The Form Sees a Rate, Not a Volume
A wall form does not respond to the number of cubic yards ordered. It responds to how many vertical feet of concrete arrive per hour while the material underneath is still fluid. Fresh concrete behaves as a liquid until stiffening gives it enough internal shear resistance to carry part of its own weight; above that transition the sheathing takes full head, below it the load tapers off and then holds steady. The height of that transition rises and falls with placement speed. Slow filling keeps the fluid zone shallow and the peak load modest. Fast filling deepens it until, at the limit, the whole lift presses on your panels as a column of liquid.
Every downstream decision hangs off that one relationship. Sheathing thickness, stud spacing, waler size, tie capacity, tie pattern, and the sequence the crew works to on the day all resolve once you commit to a placement rate and the pressure it produces. Work in the other direction — build the form first, then discover on pour day that the pump can outrun it — and you are relying on luck and a lot of hand signals across a noisy deck.
The failure is not gradual. A tie pulling through a waler, a stud buckling, a wedge bolt riding up in its slot: the form opens, the concrete leaves, and the crew has perhaps two minutes to decide whether to keep pouring into a moving wall or stop and lose the joint. The recovery runs to chipping, patching, re-tying, re-plumbing, and a cold joint the structural engineer has to accept in writing. The pressure that caused all of it was decided the moment someone chose a rate.
Geometry First: Plan Area, Contact Area, and the Rate You Can Hit
Two areas govern, and they are not the same one. Plan area — the horizontal cross-section the concrete fills — converts delivery in cubic metres or cubic yards per hour into vertical rise per hour. Contact area — the surface the concrete touches — sets sheathing sheets, panel counts, tie numbers, release agent and stripping labour. A 200 mm (8 in) wall has tiny plan area and enormous contact area, which is exactly why thin walls climb faster than crews expect while consuming more hardware per cubic metre than anything else on the job.
Take both off the drawing before anyone argues about pressure. Deduct openings honestly: a large door buck reduces plan area and speeds the rise on either side of it, and blockouts disturb tie layout far more than they disturb sheet count. Record the pour height as it will actually be placed rather than as the wall is dimensioned — a wall poured in two lifts with a construction joint is two smaller pressure problems, while the same wall poured full height in one go is one large one.
Delivery capacity supplies the other half of the arithmetic. Pump output, line size, boom reach, truck rotation and the plant's dispatch interval put a ceiling and, more often, a floor under the achievable rate. A crew that cannot keep the vibrator moving because trucks are twenty minutes apart is pouring slowly whether it intended to or not, and slow is generous to the form but hard on the joint.
Rate is delivery divided by plan area and hardware follows contact area, so both numbers have to come off the same takeoff before any pressure figure means anything.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The length of the slab or footing being formed.
The width of the slab or footing.
The height of the form boards, matching the slab's edge thickness.
Extra board length for cut ends, corner laps and stakes ripped from offcuts.
Form boards needed
7 x 8 ft boards
This estimates edge-form board footage only — it doesn't include stakes, bracing, or release agent, and assumes a simple rectangular pour with no interior forms or curves.
- Formwork surface area
- 15.33 sq ft
- Perimeter (with waste)
- 50.6 linear ft
They open the calculator with your figures already in it
Concrete Formwork Calculator: 7 x 8 ft boards — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Rectangles only. The perimeter is taken as 2 x (length + width), so L-shapes, curved edges, steps, blockouts and any interior or divider forms are not counted. Break an irregular pour into rectangles and add the runs together.
- One course of boards. The count divides the perimeter by a single stock length (8 ft, or 2.4 m on the metric setting) and never doubles it, so if the form height is taller than the width of the board stock you actually buy, the second course is on you.
- Perimeter boards only. Stakes, kickers, braces, walers, ties, screws and form-release agent are not counted, and the cutting allowance covers waste on the run rather than any of that hardware.
- This is a take-off, not a formwork design. Nothing here checks whether the boards and stakes will hold the lateral pressure of wet concrete, which depends on pour rate, mix, slump and temperature. For anything deeper than a slab edge, bracing and tie spacing are a separate check.
- Ground-supported edge forms only. There is nothing for the decking, joists, stringers or shoring under a suspended slab, and nothing for excavation, screed rails or subgrade preparation.
Turning Rate Into a Design Pressure
Design pressure comes from placement rate, concrete temperature, unit weight, and what the mix is doing chemically while it sits. ACI 347R, Guide to Formwork for Concrete, carries the rate-of-placement relationships used across North America for walls and columns, together with a lower bound you may not design below and an upper bound at full liquid head. Those relationships come with qualifying conditions attached — slump range, vibration method and depth of immersion, and the admixtures present. Step outside them and the guidance sends you straight back to full hydrostatic pressure, which is a much larger number and a much heavier form.
Elsewhere the governing document changes and so do the numbers. CIRIA Report 108, Concrete Pressure on Formwork, underpins UK practice alongside BS 5975, Code of Practice for Temporary Works Procedures and the Permissible Stress Design of Falsework. EN 12812, Falsework — Performance Requirements and General Design, applies across much of Europe; AS 3610, Formwork for Concrete, in Australia; CSA S269.1 for falsework and formwork in Canada. They do not agree in detail, and the project specification rather than habit decides which one applies. Check the specification and the temporary works design before importing a pressure figure from a job in another country.
What comes out is an envelope, not a single number. Pressure builds with depth to a maximum and then holds roughly constant through the lower part of the form, because the concrete down there has already stiffened enough to carry itself. Ties near the base frequently carry less than ties a metre or two (three to six feet) above them. Designers who miss that put their tightest tie spacing at the bottom and leave the peak-pressure band underserved.
This is the point where an agreed rate and an expected placement temperature stop being pour-plan talk and become the pressure your panels, walers and ties actually have to survive.
The fresh concrete's unit weight.
How fast the concrete surface rises at this form location, in feet per hour.
The temperature of the fresh concrete at placement, in °F.
ACI 347R Table 2.2 chemistry coefficient for the cement/admixture combination used.
The total vertical height of the wall being poured.
Maximum lateral formwork pressure
970 psf
Wall pressure is rate-limited rather than height-limited. Because a wall is filled over a long period, the concrete at the base normally stiffens before the top arrives, and the pressure envelope caps out well below full hydrostatic.
- Unit weight coefficient Cw
- 1 (dimensionless)
- Full hydrostatic ceiling
- 1,500 psf
They open the calculator with your figures already in it
ACI 347 Wall Formwork Rate-of-Placement Pressure Calculator: 970 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 970 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The rate-based reduction depends on the concrete stiffening as expected. Retarders, cold weather and high cement replacement all delay that and raise the pressure.
- Excludes wind load on the form face, which on a tall free-standing wall form can govern the bracing even though it does not affect the ties.
- Does not address the uplift and lateral load at a construction joint, or the loads from the placing equipment resting on or against the form.
Temperature, Mix and Admixtures Move the Same Number
Cold concrete sets slowly, stays fluid longer, and therefore presses harder. The same wall at the same rate, poured at 5 °C (41 °F) instead of 25 °C (77 °F), is a materially different form design; the gap is wide enough to change tie spacing and occasionally sheathing thickness. Early-morning winter pours and mass placements in shaded lift shafts routinely produce the worst case on a project, and they are the pours least likely to get a fresh pressure check because the form was designed in summer.
Retarders, water reducers and high-range admixtures all extend the fluid window in the same direction. Slump matters, and so does the placement method: internal vibration reaching deep into a lift re-liquefies concrete that had begun to stiffen and resets the transition downward. Revibrating to close a lift line is a legitimate finishing decision and a pressure decision at the same time, which puts it in the pour plan rather than in the vibrator operator's discretion.
Self-consolidating concrete sits outside the rate-based rules altogether. With little yield stress at placement it can approach or reach full liquid head no matter how slowly it is placed, and thixotropic recovery varies from mix to mix. Treat an SCC wall as a hydrostatic problem unless the supplier and the temporary works designer can substantiate something lower. Watch for the quiet version of this failure too: a supplier switching to a more flowable mix to help placement changes the load on the form without anyone signing a drawing.
Where a flowable mix removes the rate from the argument, unit weight and full pour height are all that is left, and seeing that figure beside the rate-based one is what makes the difference argue for itself.
The full vertical height of the SCC pour against the formwork.
The unit weight of the fresh SCC mix.
Design lateral pressure
1,500 psf
This is the conservative full-hydrostatic design value ACI recommends by default for SCC. A qualified formwork engineer may justify a lower design pressure using documented field pressure measurements for your specific mix and placement method — real measured pressures have been reported as low as 34-60% of full hydrostatic in some studies, but full hydrostatic remains the required default absent that data.
- Equivalent in psf
- 1,502.32 psf
They open the calculator with your figures already in it
SCC Formwork Lateral Pressure Calculator: 1,502 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1,500 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This is the pressure demand only, not a formwork design. It says nothing about whether the sheathing, studs, walers, ties or bracing can carry the figure, and it does not set tie or waler spacing — those checks belong to whoever is responsible for the formwork design.
- The answer is the peak pressure at the base of the pour. It does not give the pressure distribution up the form, the total lateral force per unit length of wall, or the height at which the resultant acts — which is what a tie and waler layout is actually set out from.
- Concrete pressure is the only load included. Wind on the form face, the placing crew and their equipment, impact and surge from discharge, uplift at the base, and the out-of-balance load on a single-sided or sloping form are all excluded. On many forms it is one of those, not the concrete, that governs the bracing.
- Bottom-injection placement is outside this model. Pumping SCC in through a port at the base of the form adds the pump's own pressure to the hydrostatic head, and the pressure in the form can then exceed unit weight times height.
- The two inputs carry no information about rate of placement, concrete temperature, rest period between lifts, form geometry or the mix's thixotropy. The result cannot reflect a mix or a placement method that behaves differently from a static liquid, in either direction.
The Load Path Down: Sheathing, Studs, Walers, Ties
Load walks down a chain and every link carries its own spacing. Sheathing spans between studs, studs span between walers, walers span between ties, and ties carry the whole lot back through the concrete to the opposite face. Each element is governed by the pressure at its own height, which is why a properly designed wall form tightens tie spacing through the peak band and opens up above and below it instead of running one spacing top to bottom.
Deflection often controls before strength does, particularly on architectural concrete with a specified formed-face flatness. ACI 117, Specification for Tolerances for Concrete Construction and Materials, together with the project documents, sets what the finished wall must achieve. A panel that is strong enough but bellies between studs produces a wall that passes on strength and fails on tolerance, and grinding a bellied architectural face is not a repair anyone accepts twice. Plywood stiffness also varies with face grain orientation — sheets laid the wrong way lose a large share of their span capacity for no visible reason.
Ties deserve separate attention. Rated working loads govern, never ultimate loads; the margin between them absorbs impact from a surging pump line, prying from misaligned walers, corrosion on reused hardware, and the reality that no tie sits perfectly perpendicular to the face. Mixing tie systems within a single wall, or reusing coil rod with damaged threads, turns a designed load path into a lottery. Count ties against the layout drawing before the truck arrives, not while the wall is filling.
The chain that carries the push
- Sheathing — spans stud to stud and often fails on deflection before strength, and loses a large share of that span if the face grain is turned the wrong way Plywood and OSB Sheet Calculator (Subfloor, Wall and Roof)
- Studs behind the face — spacing answers whatever the concrete is pushing at that level, so the band through the peak wants members set closer together than the band above it
- Walers — collect the studs and hand the load to the ties; a waler out of line pries on the tie beside it rather than sharing with it
- Ties through the wall — the only member sized on a rated working load rather than an ultimate one, and the reason one tie system per wall is a rule and not a preference Concrete Formwork Tie Spacing Calculator
A tie's tributary area falls out of its rated load and the pressure at its own height, which is what turns one spacing top to bottom into a tighter grid where the peak sits.
The design lateral pressure the formwork must resist.
The tie's manufacturer-rated safe working load.
The safety factor applied to the tie's rated capacity.
Maximum tie spacing
1.47 ft
This assumes a uniform square grid and uniform pressure across the form face — real formwork design must also check the sheathing and stud/waler capacity between ties, not just the tie spacing alone.
- Allowable tie load (after safety factor)
- 2.25 kips
- Tributary area per tie
- 2.15 sq ft
They open the calculator with your figures already in it
Concrete Formwork Tie Spacing Calculator: 1.47 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Only the ties are checked. The sheathing's bending and deflection, and the stud and waler spans between ties, are separate calculations, and on ordinary ply-and-timber forms one of those usually governs before the tie capacity does. Treat this spacing as a ceiling, not a layout.
- One pressure is applied over the whole form face. Pressure builds with depth, so a grid set from the base figure is conservative near the top, and a grid set from an average over the height is unsafe at the bottom, which is where a form bursts. Nothing here zones the wall or opens the grid up as it rises.
- The tributary area is that of a tie in the middle of a continuous grid. Ties at the ends and edges of a panel, at corners, and around blockouts and penetrations pick up more than their square share, and no allowance is made for them. It also ignores where a system panel's tie holes and walers actually fall, so the buildable spacing is the next hole position below this figure.
- The capacity you enter is the tie's, not the assembly's. Waler brackets and bearing plates, the cone, washer or wedge at the form face, thread engagement on she-bolts and taper ties, and a tie anchored back into a previous pour can each be weaker than the tie itself, as can a tie that has been reused, bent or corroded.
- Only the concrete's lateral pressure is resisted here. Wind on the form face, surcharge and impact from placing equipment, over-vibration, and eccentric or single-sided pours are carried by the bracing and by the form's own design, and none of them enter this arithmetic. This is a check on one component, not a formwork design — that belongs to a competent formwork designer.
Bracing Carries What the Pressure Doesn't
Concrete pressure is only the load you can calculate. Bracing answers everything else: wind on an erected panel standing empty, eccentric load from a pump line surging, personnel and equipment on the walkway, a reversing truck nudging a kicker, and the minimum lateral force ACI 347R requires forms to resist whatever the wind map says. ASCE 37, Design Loads on Structures During Construction, is the construction-stage load reference on many US projects and is worth reading before assuming the permanent-works wind case covers you.
Braces work in tension and compression and must be anchored to something able to take both. A kicker footed on loose fill, a screw jack bearing on a slab poured yesterday, or an anchor drilled into a slab edge without any check on pullout in green concrete are all routine sights and all avoidable. Set braces within the angle range the manufacturer's data permits — flatter braces multiply the force in the brace and in its anchorage — plumb the form before the pour, and check plumb again while it fills, because pressure that is not perfectly symmetrical will walk a form quietly.
Single-sided forms against existing structure change the problem entirely. With no opposite face to tie back to, the full lateral load lands in the bracing frame and its anchorage, which calls for a designed solution and an anchor test regime rather than an assembled guess. OSHA 29 CFR 1926 Subpart Q, Concrete and Masonry Construction, requires formwork in the United States to be designed, erected, supported, braced and maintained to carry the loads imposed on it; equivalent duties sit in other jurisdictions' construction regulations.
Holding the Rate on the Day
The rate written on the drawing has to survive contact with the pour. Give the gang a lift height and a clock: mark the form at lift intervals in crayon or tape, and write beside each mark the time it should be reached. A leader who can see that the wall is running ahead of the schedule can throttle the pump before pressure exceeds what the form was designed for, which is a far better position than reacting to a panel that has already opened.
Pouring in horizontal lifts rather than chasing the wall end to end keeps the rise even and the load symmetrical. Lift depth should suit the vibrator's effective immersion so each layer knits into the one below without over-working the layer beneath. Where the plan calls for a deliberate pause to let the base stiffen, hold it long enough to change the concrete's state and record when it happened — a pause of a few minutes buys nothing except a visible lift line.
Weather and delivery break more pour plans than design does. A truck that fails to show stretches the pour and threatens a cold joint; three trucks arriving together tempt a crew to place them all and blow the rate outright. Settle in advance who holds authority to stop, and where the fallback joint goes, because that call made at the wall head with concrete waiting is reliably the wrong one.
Reading the Form While It Fills
A form talks before it fails. Grout weeping at a panel joint means the joint has opened; grout at a tie means the tie or its cone is moving. Dust shaking off a waler bolt, a visible bulge along a stud line, a brace gone slack, a wedge creeping up its slot — these show up minutes ahead of the failure they precede, and every one of them is visible to somebody who is looking.
Assign one person to walk the form during the pour with no other duty, covering the back face and underneath wherever the form spans an opening. A torch beats no torch even in daylight. Clear the walking route before concrete starts, since a form you cannot reach is a form you cannot inspect, and a deck crowded with hoses means the first sign of trouble reaches you as a noise rather than an observation.
Should pressure climb past plan, the intervention is the rate, not the hardware. Slowing placement lets the base stiffen and drops the peak; adding ties into a loaded form seldom helps and often makes matters worse by prying on a waler already under load. Stop, wait, resume at a slower rise, and treat the resulting line as a planned joint rather than a defect to hide.
Stripping and Reshoring: the Rate Question Reversed
Removal runs on the same clock in reverse — strength gained over time, measured rather than assumed. Vertical forms carrying nothing but lateral pressure can come off once the concrete holds its shape and arrises without damage. Forms supporting load stay until strength is demonstrated by field-cured cylinders, maturity, or whatever method the specification names. ACI 301, Specifications for Structural Concrete, along with the project documents, governs the decision, and the structural engineer's release is not a formality to be collected afterwards.
Shoring and reshoring on multistorey work spreads load across floors that are all young at once; ACI 347.2R, Guide for Shoring/Reshoring of Concrete Multistory Buildings, sets out how the sequence is meant to work. Stripping a level early to keep the form cycle turning shifts load onto slabs that may not be ready for it, and the resulting crack pattern surfaces months after the crew has moved on.
Recovered material becomes next week's pressure capacity. Panels dropped from height, plywood with delaminated faces, kinked walers and stretched tie rod all reduce what the next assembly can take, and the next crew will assume the form is as-designed because it looks as-designed. Grade the material as it comes down, paint the scrap, and keep tie hardware sorted by system so nobody mixes two of them into one wall.
Where the specification accepts maturity instead of a broken cylinder, the index accumulated so far is the number that either releases a supported form or tells the gang to leave it standing another shift.
The average concrete temperature over the elapsed curing period, in °C.
The total elapsed curing time, in hours.
The temperature below which the mix gains no useful strength. ASTM C1074 recommends 0°C.
Maturity index
3,360 °C-hours
This uses a single average temperature for simplicity — a real maturity study integrates continuously logged temperature data over time, which is more accurate when temperature varies significantly during curing (e.g. cold nights, hot days). The datum belongs to your mixture: 0°C is ASTM C1074's recommendation for plain Type I cement, and Annex A1 sets out how to measure it for anything else.
- Elapsed time
- 7 days
- Datum temperature used (°C)
- 0 °C
- Temperature above datum (Δ°C)
- 20 Δ°C
They open the calculator with your figures already in it
Concrete Maturity Method Calculator (Nurse-Saul): 3,360 °C-hours — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- A maturity index is not a strength. Until a strength-maturity relationship has been developed for the mix actually supplied — cylinders from that mix, broken at several ages against their own logged maturity — this number cannot be read as an in-place compressive strength, and on its own it does not release formwork, tendons, sawcutting or traffic.
- Nurse-Saul sums temperature over successive intervals; this takes one average for the whole period instead. Cold nights, the hydration peak and any spell the concrete spent below the datum are all flattened into a single figure, so the wider the temperature swing over the period the further this sits from what an embedded logger would report.
- It covers one temperature, not a position in the pour. Corners, edges, thin sections and the top surface run colder than the core and reach a given maturity later, and a striking decision belongs to the coldest critical location rather than to an average of the element.
- The datum is taken exactly as entered — nothing here checks it against your cement, admixtures or supplementary cementitious materials. The Nurse-Saul function is also linear in temperature, so it does not capture the reduced ultimate strength of concrete cured hot: a warm pour can accumulate maturity quickly and still fall short at later ages, which is what the Arrhenius equivalent-age methods exist to address.
- At or below the datum the index reports zero. That is the method's bookkeeping convention, not a verdict on the concrete: it says nothing about whether a pour that got cold or froze has been damaged, and it does not cover cold-weather protection requirements.
- The index is reported in °C-hours whichever measurement system you are reading the site in, and the two temperature rows beneath it are shown in Celsius terms for the same reason: the figure is only meaningful against the strength-maturity curve it is compared to, and that curve carries the units its own cylinders were logged in. Enter your temperatures in whichever units you measured — those fields convert — but if your calibration curve is in °F-hours, multiply this figure by 1.8 before reading a strength off it.
Before the first panel goes up
Six things settled on paper turn a pour rate into a form that survives it — none of them are decisions to make at the wall head with concrete on the way.
- Plan area and contact area, taken separately — One converts delivery into vertical rise per hour; the other sets sheets, ties, release and stripping labour.
- Design placement rate, as vertical rise per hour — Agreed with the pump operator and the batch plant, not back-calculated from the pour volume afterwards.
- Expected concrete temperature at placement — The cold end of the forecast governs — lower temperature keeps concrete fluid longer and raises the peak.
- Tie system, rated working load and count — One system per wall, hardware inspected for thread and bend damage, count checked against the layout drawing.
- Brace anchorage capacity in the supporting slab — Verify pullout in concrete that may still be green, and hold to the brace angle the manufacturer's data permits.
- Pour plan with lift marks and clock times — Marks on the form give the gang a way to see the rate drifting before the form starts showing it.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.