The drop is a budget, and it is already being spent
A harness does not stop a fall. It ends one, some distance below where it began, and that distance was fixed by decisions taken long before anybody stepped over the edge. Working a leading edge, a scaffold return, a roof perimeter or an unformed deck penetration, one number matters more than any other: how much air sits between the attachment point on a worker's back and the first hard thing underneath. Every component in the system is a claim on that number.
Six line items spend it. Free fall, before the energy absorber does any work at all. Deceleration, while the absorber tears out. Harness stretch and the dorsal D-ring riding up the back. The length of the person hanging below that D-ring. Movement in the anchor and in whatever the anchor is fixed to. Then a margin at the bottom, which exists so the sum can be slightly wrong without being fatal. Add the six, compare against the measured drop, and the answer is either yes or a different system.
Ownership of those line items is scattered across the job. Procurement chose the lanyard, the rigger chose the anchor height, the temporary works designer signed the beam, and the groundworks crew left the surface the scaffold legs now stand on. None of them see the total. The operative clipping on does, usually with a poor view of the drop and no tape in his pocket.
Permitted free fall and required margin vary by jurisdiction. United States construction work sits under OSHA 29 CFR 1926 Subpart M, Fall Protection, with equipment requirements in the ANSI/ASSP Z359 series. European component standards include EN 355 for energy absorbers, EN 360 for retractable type fall arresters, EN 795 for anchor devices and EN 13374 for temporary edge protection systems. Australia and New Zealand work to AS/NZS 1891.4 for the selection, use and maintenance of industrial fall arrest systems. The arithmetic behaves identically everywhere; the values you are permitted to substitute into it do not, and the regulation governing the site decides which apply.
Free fall: the term you set with a tape
Free fall is the distance travelled before the absorber begins doing anything, and it is set with a tape rather than a calculator. Anchor overhead, connector short, no slack in the line: the term approaches zero. Anchor down at foot level with a two metre lanyard: the worker falls the length of the lanyard to reach the anchor, then the same length again before the pack opens. Two decisions about where to clip, separated by a couple of metres of steel, produce fall distances that differ by a factor of four.
Slack hides in the connectors. A beam anchor sling, an oversized rebar hook, an extra carabiner added because the first one would not close on the ring, a rope grab that has to travel before it bites, a lanyard clipped back to itself around a member — each contributes real centimetres, and they accumulate in one direction only. Crews rarely measure them, and the assembled system is routinely longer than the length printed on the lanyard.
Components are qualified for a stated free fall. Standard energy-absorbing lanyards are generally rated for a free fall no greater than about 1.8 m, with equipment marked for longer free fall sold as a separate product class. Exceeding the rating does two things at once: the absorber deploys further than the figure in your arithmetic, and the arrest force may climb beyond what the harness and anchor were qualified against.
What the absorber takes back
Absorber deployment is the only line item deliberately designed to consume distance. Its job is to convert a short violent stop into a longer, survivable one, capping the arrest force transmitted into the body and the anchor. That cap is a regulated number, and the price paid for it is stretch: the pack tears, elongates and adds length to the system exactly when the worker can least afford it.
Deployment length is not a single figure. It varies by standard, by the rated free fall of the product, and by the mass being arrested; a heavier operative with a full tool pouch deploys more absorber than a light one. The label carries the rated capacity range and the maximum deployment figure that belongs in your sum, and catalogue memory does not.
Self-retracting devices change the shape of the problem rather than eliminating it. They lock quickly, so free fall is small, but arrest distance still exists, and devices qualified for use over an edge behave differently again because the line drags across the edge before and during arrest. A device marked for leading-edge use carries its own arrest distance and its own minimum clearance, sometimes larger than a crew expects from something that looks like an inertia reel. Anchoring one below the D-ring, which is the whole point of a leading-edge unit, also reintroduces free fall that an overhead installation would not have had.
Anchors move, and the movement is distance
An anchor that moves spends clearance as surely as an absorber that deploys. A horizontal lifeline sags at midspan under arrest load, and that sag drops the worker further while multiplying tension into the end anchors; a long span with few intermediate supports can add more distance than the absorber does. Manufacturers of flexible line anchor systems declare a deflection figure for a given span and number of users, and that declared figure, not an assumption, belongs in the sum.
Rigid structure deflects too. An anchor bolted to a purlin, a light beam, a metal deck or a proprietary roof anchor plate loads that member with an arrest force in the kilonewton range applied in a fraction of a second. Roof anchors fixed into decking, insulation or corroded sheet can pull, tilt or dish the substrate before they hold, and each millimetre of that movement is distance the worker travelled. Anchor device standards such as EN 795 classify devices by type and require the supporting structure to be verified separately — the anchor being certified says nothing about the purlin being adequate.
Sharing matters too. A second worker arriving on a line rated for one changes both the deflection and the load path, which is why anchor selection is a temporary works decision with a designer's name against it.
An anchor's rating is a claim until someone pulls it, and the load to hold and the number to sample come from the lot, not from the label.
The tension the anchor is designed to carry in service.
How many times the design tension the specification asks the test to reach.
How many anchors this lot covers — one installer, one product, one condition.
The proportion of the lot the specification requires to be tested.
The floor the specification puts under the sample, whatever the percentage gives.
Proof load to apply and hold
2,700 lbf
The sample rate governs this lot. A proof test demonstrates that the installation holds the specified load without measurable displacement; it is not a capacity test and must not be taken to the anchor's ultimate. Confirm the specification's cap on the applied load, and confirm what counts as a failure and what happens to the rest of the lot when one occurs.
- Anchors to test in this lot
- 6 anchors
- Sample size before the specified floor
- 6 anchors
- Load applied above the design tension
- 899.24 lbf
- Anchors covered by each tested anchor
- 20 anchors
They open the calculator with your figures already in it
Concrete Anchor Proof Load Test Calculator: 2,698 lbf — 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 — 2,700 lbf — 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
- Does not set the multiplier or the sample rate. Both are contract values, and a page that supplied them would be inventing the specification.
- Does not check that the proof load is safe to apply. A multiplier taken against a load close to the anchor's capacity can exceed the steel or the concrete during the test itself.
The ground under the legs
Everything measured from a scaffold platform assumes the scaffold has not moved. Standards bear onto base plates, base plates bear onto sole boards or onto whatever the ground crew left, and the load path ends in soil whose bearing capacity nobody on the erection gang has checked. Soft made ground, a backfilled service trench under one leg, a saturated verge after a week of rain, or a plate resting half on slab and half on fill will settle. Uniform settlement lowers the whole platform and quietly shortens the drop, which sounds harmless until the guardrail geometry, the tie loads and the standard's plumb all change with it.
Differential settlement is the dangerous version. One leg drops, the frame racks, the platform slopes, the toe board opens a gap and the anchor mounted to a standard is no longer where the designer put it. A scaffold that has moved is a scaffold whose edge protection and whose anchor points both need re-checking before anyone clips on, and the movement is usually visible first at the base plate rather than at the platform.
Checks at the base are quick and worth doing every time the perimeter changes. Plates fully seated with no rocking, no packing with brick, offcuts or timber wedges, sole boards spanning across soft spots rather than sitting inside them, and a look at whether groundworks have excavated near a leg since the last handover. Where the ground is questionable, the bearing pressure under the plate is the number to calculate rather than eyeball.
Every clearance figure taken from a scaffold platform assumes the legs have not sunk, so the pressure under the base plate belongs at the same moment you write the drop on the permit.
The load carried down through a single scaffold leg.
The actual ground-contact area under the leg — the base plate alone, or the base plate plus a sole board if used.
The allowable ground bearing capacity at this location.
Applied bearing pressure
2,730 psf
This is a first-pass bearing-pressure screening only (leg load ÷ contact area, vs. a user-supplied allowable value). It does NOT verify overall scaffold stability, tie-in/guying requirements (OSHA 1926.451(c)(1), 4:1 height-to-base rule), component structural capacity (1926.451(a)(1), 4x max intended load), sole-board bending/shear adequacy, eccentric or dynamic loading, or site soil variability. Allowable bearing capacity must come from your own geotechnical data or a competent person's site assessment — OSHA does not publish a universal bearing value. Per 1926.451(a)(6) and (f)(7), scaffold design and erection must be under a qualified/competent person; this calculator is not a substitute for that.
- Allowable bearing capacity (user-supplied)
- 3,132.82 psf
They open the calculator with your figures already in it
Scaffold Base Plate Bearing Pressure Calculator: 2,726 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 — 2,730 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
- Treats the leg load as concentric on the plate. A leg out of plumb, or a plate not bearing flat, concentrates the load on one edge and the real peak pressure is higher than load ÷ area — which is the case that punches a sole board into soft ground.
- Assumes the full contact area bears. A base plate on uneven ground, or a sole board that has begun to bridge, carries on part of its area, and the figure you entered is the plate's size rather than what is actually in contact.
- Static load only. Nothing here accounts for material being landed on a lift, wind uplift, or the out-of-balance loads present while the scaffold is being erected or struck.
- Ground does not stay as it was measured. An allowable bearing figure taken in dry weather is not the figure after a week of rain, after a trench is opened alongside, or over a backfilled service.
Swing fall: a horizontal error with a vertical bill
Swing fall is a horizontal error that arrives as a vertical bill. Work well to one side of the anchor and the fall becomes a pendulum: the worker drops, then swings, and the lowest point of that arc sits below the point at which the system arrested him. Required clearance rises with the offset, and the arithmetic that closed comfortably directly under the anchor may not close at all thirty degrees away from it.
Impact is the second cost. A swinging body travels fast and horizontally into columns, formwork, stacked material, scaffold standards and the face of the structure itself, and the injury from that strike does not care that the harness held. On a leading edge the line also drags sideways across the edge under load, which is precisely the condition that leading-edge rated devices are tested for and standard ones are not.
Limiting offset is a planning job, not a discipline problem. Move the anchor, add anchors, or run a horizontal line with a travelling trolley so the attachment tracks the work. Telling a crew to stay within a cone while the work plainly extends outside it produces exactly the shortcut everyone predicted.
A guard that meets height and load is the only arrangement on this page that needs no fall-clearance arithmetic at all, which is why it is worth checking first.
The measured height of the guardrail's top edge above the walking/working surface.
The manufacturer-rated (or engineered) load capacity of the top rail.
The manufacturer-rated (or engineered) load capacity of the midrail or other structural member.
Top rail height
42 in
Measured against the three fixed figures in OSHA 29 CFR 1926.502(b), which this page restates and does not interpret. The top rail height entered is inside the 39–45 in (991–1,143 mm) the rule fixes. The top rail's rated load is at or above the 200 lbf (890 N) minimum, and the midrail's is at or above the 150 lbf (666 N) minimum. Post spacing and anchorage, screening between the top rail and the walking surface, and the 21-inch wall exemption are not examined here. Matching the figures quoted is not compliance. The rest of the requirement, and the installed work, are outside what this page can see.
- Top rail rated load capacity
- 200 lbf (OSHA min 200)
- Midrail rated load capacity
- 150 lbf (OSHA min 150)
They open the calculator with your figures already in it
Guardrail System Fall-Protection Compliance Checker: 42 in — 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 — 42 in — 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 height check reads the rail standing still. OSHA also requires that when the 200 lbf load is applied downward, the top edge must not deflect below 39 inches — 1926.502(b)(4) — so a rail installed at 39 or 40 inches passes the height test here and fails the standard the moment somebody leans on it. Height and stiffness are one requirement together, and only the height half is checked above.
- Only the midrail's rated LOAD is compared; its height is not. The rule puts the midrail midway between the top edge and the walking surface — roughly 21 inches under a 42-inch rail — and a midrail sitting low leaves an opening a body can pass through while both load figures on this page sit inside the values they are compared with.
- Nothing above looks at what the rail is made of. OSHA bars steel banding and plastic banding as top rails or midrails outright, sets a quarter-inch minimum nominal diameter or thickness, requires wire-rope top rails to be flagged at intervals no greater than 6 feet, and requires the surface to be finished so it cannot cut a hand or snag clothing. A rail can carry 200 lbf and still be non-compliant on any one of those.
Spending nothing: the passive edge
The cheapest way to win the arithmetic is to make the total zero. A compliant guardrail, a secured cover over a penetration or a properly installed net removes the fall rather than managing it, and no clearance calculation is needed for a fall that never starts. Temporary edge protection systems in Europe are classified by EN 13374 according to the loading and the slope they are intended for, and United States guardrail geometry and load requirements sit within OSHA 29 CFR 1926 Subpart M. Class, height, infill and load path all have to match the actual edge rather than the one in the standard detail.
Passive protection still deflects, which is the part crews forget. A rail loaded by a body, a wheelbarrow or plant movement moves outward before it holds, and how far it moves is governed by post spacing, post fixing or embedment, and rail stiffness. Barriers along a highway works interface or a site perimeter behave the same way: increase the spacing between posts and the system becomes more flexible, deflecting further under the same impact. A barrier that deflects past the edge it is protecting has spent distance it never had.
Detailing decides whether the edge stays at zero. Toe boards to stop material becoming a falling object, no unguarded gaps at returns and terminations, and a written rule about who is allowed to remove a panel for a load-in and who signs it back. Most guardrail incidents involve a section that was legitimately removed and never reinstated, not a section that failed under load.
Post spacing is the variable that decides how far a barrier travels before it holds, so it belongs at the exact point where you are deciding whether a passive edge really spends zero distance.
The total length of the W-beam guardrail run.
The center-to-center spacing between guardrail posts.
Guardrail posts needed
54 posts
They open the calculator with your figures already in it
Highway Guardrail Post Spacing Calculator: 54 posts — 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
- The run length is an input here, not an output, and it is the number that decides whether the barrier works at all. Length of need comes from the hazard's offset from the traveled way, the design speed and traffic volume, and the runout length tables in the Roadside Design Guide. A barrier that stops short lets a vehicle round its end and reach the hazard it was installed to shield.
- Every post counted is a line post. A crashworthy end terminal is a proprietary assembly with its own posts, its own post types — breakaway timber in soil tubes, or steel posts in foundation sleeves — and its own spacing across the first 37.5 to 50 ft (15 m) of the run. On a run terminated at both ends, a sizable share of this count is neither the post nor the spacing being priced.
- Post count says nothing about post length or embedment, and embedment is what lets a post yield rather than tear out of the ground. Depth is set by the soil and by the conditions a standard drawing does not assume: a post behind a curb, on a fill slope, or through a concrete mow strip is a different detail, often a longer post or a leave-out. Correct spacing over insufficient embedment gives the rail nothing to work against.
When the sum does not close
Sometimes the sum does not close, and the honest response is to change the system rather than the assumptions. Raising the anchor overhead is the single most effective move available, because it collapses the free fall term and often the swing term at the same time. Swapping an energy-absorbing lanyard for a self-retracting device anchored overhead does the same thing with less deployment. Both are cheaper than discovering the shortfall at the bottom.
Restraint beats arrest wherever geometry permits it. Configure the anchor position and lanyard length so the D-ring physically cannot reach the edge, measure that distance on site rather than assuming it, and remember that restraint is only restraint while the anchor stays where you left it and the lanyard stays the length you specified. A restraint system that reaches the edge is an arrest system with no clearance behind it.
Where neither works, the answer moves up the hierarchy: a mobile elevating work platform, a proper working platform with edge protection, a net installed close beneath the work, or a resequenced task done from the deck below before the edge exists. Rescue planning belongs to the same decision, because tolerance for suspension is limited and a successful arrest still ends with someone hanging.
Toe boards are the part of an edge system that gets counted last and runs short first, because the run follows every return and every opening.
The total linear length of platform/guardrail edge that requires a toe board.
The length of the toe board stock/lumber as supplied.
The vertical height of the toe board as installed.
The clearance gap between the bottom edge of the toe board and the walking/working surface.
Toe board stock lengths needed
17 board lengths
Measured against the two fixed figures in OSHA 29 CFR 1926.502(j), which this page restates and does not interpret. The board height entered is at or above the 3-1/2 in (9 cm) minimum. The gap above the walking surface is at or below the 1/4 in (0.6 cm) maximum. The 50 lbf (222 N) load capacity in the same paragraph is confirmed with the board's manufacturer and its material, not calculated here. Matching the figures quoted is not compliance. The rest of the requirement, and the installed work, are outside what this page can see.
- Toe board height
- 3.5 in (OSHA min 3.5in)
- Gap above surface
- 0.2 in (OSHA max 0.25in)
They open the calculator with your figures already in it
Guardrail Toe Board Height and Quantity Calculator: 17 board lengths — 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
- Checks the board and stops at the top of it. Where tools or material are stacked higher than the toeboard's top edge, the falling-object rules call for paneling or screening carried up from the surface or the board to the toprail — so a compliant 3-1/2 in board with pipe stacked a foot high behind it protects nobody underneath and is still a violation.
- Assumes an unbroken run along the edge measured. The board only works where it is fastened tight against the edge it protects: set back from the deck edge it leaves a slot that passes exactly what it was there to stop, and ladder access points, gates and hoist openings need their own removable protection rather than a gap in the run.
- Reads against the fall-protection rule for guardrail systems. A scaffold platform is governed by the scaffolding standard instead, and its toeboard provision is stricter on load — 100 lbf rather than the 50 lbf quoted on this page, on the same 3-1/2 in minimum height. Confirm which rule the platform falls under before treating the comparison here as the whole answer.
Re-walking the arithmetic every shift
Numbers written on Monday describe Monday's site. The drop below an edge changes as decks are struck, material lands, scaffolds rise, skips arrive and excavations open, and each of those changes rewrites a line in the arithmetic without anybody being told. Re-measuring the drop is a two minute job at the start of a shift and belongs in the same conversation as the permit, not in a file.
Record what was actually assumed rather than a generic figure. The anchor used, its height relative to the D-ring, the assembled connector length, the absorber's rated deployment from the label, the declared anchor deflection and the measured drop give a chain anyone can audit later. Entries reading "harness and lanyard" describe nothing and defend nobody.
Near misses in this trade read deceptively well. The system worked, the worker was arrested, everybody went home — and the boots stopped a hand's width above a stack of scaffold boards nobody had included in the sum. Treat that as the failure it is, because the margin that saved it was the last line item in the budget, and it was never meant to be spent.
Before the first clip-on
Six checks that decide whether the clearance arithmetic closes on the edge you are actually standing at.
- Measured drop, edge to first obstruction — Taken with a tape or laser from the working level, not scaled off a drawing, and re-taken whenever the deck below is struck or loaded.
- Absorber label rather than memory — Rated free fall, maximum deployment and the capacity range are printed on the pack; a heavier tooled-up user deploys more.
- Anchor height relative to the dorsal D-ring — Overhead anchoring cuts free fall to near zero; foot-level anchoring can double it and may exceed what the component is rated for.
- Base plates and sole boards in full contact — Packing with brick or offcuts, or a plate sitting half over a backfilled trench, turns a static platform into a settling one.
- Horizontal offset from the anchor — Working outside a narrow cone adds swing, and the low point of the arc sits below the height at which the system arrested.
- Guardrail post spacing and fixing — Deflection under load follows spacing, embedment and rail stiffness; a rail that moves outward has already spent clearance.
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.