The job
Modelled, not recorded. Every figure on this page is re-run through the calculator it names whenever the site is built; the estimate is worked, but no job was carried out, so the last section gives the mechanism of each likely error rather than a measured overrun.
A small slab looks like one quantity — the concrete — and is actually seven, most of which are decided before any concrete is ordered. This walkthrough runs them in the order they are actually done, because each layer's thickness changes the excavation depth above it and the levels below it.
The job: a 6 × 4 m ground-bearing slab for a garden room, on reasonable ground, with the finished slab level set from the existing patio. Nothing structural bears on it beyond the building itself.
THE MEASUREMENT THAT CONTROLS THE REST IS THE DIG DEPTH, and it is not a choice — it is the sum of the layers above it. Sub-base thickness plus sand blinding plus membrane plus slab thickness, measured DOWN from the finished level. Getting that sum wrong by 50 mm changes the excavated volume by nearly one and a half cubic metres on a slab this size, which changes the muck away, the haulage and the sub-base together.
THE SECOND IS THAT CONCRETE IS ORDERED IN WHOLE LOADS. Unlike tiles or boards, a shortfall cannot be topped up an hour later without a cold joint, and the excess cannot be returned. That asymmetry is why the concrete allowance is treated differently from every other material on this page.
What was measured, and how
Finished slab level, set from a fixed datum
Set 150 mm below the house DPC, falling away from the building
Taken from something that will still be there when the slab is poured — a damp-proof course, a threshold, an existing hard surface — rather than from the ground, which changes as soon as digging starts.
Slab plan size, including the formwork allowance
6.00 × 4.00 m slab; excavate to roughly 6.6 × 4.6 m for working space
The slab is the building's footprint plus whatever it oversails, and the FORMWORK sits outside that line. Excavation is to the outside of the formwork plus working space, not to the slab edge.
Existing ground level across the area
Falls about 120 mm across the 6 m length
Levels taken on a grid rather than at the corners, because the volume depends on the average and a sloping site's corners mislead in both directions.
The layer build-up, summed downward
150 sub-base + 25 blinding + membrane + 100 slab = 275 mm below finished level
Sub-base, blinding, membrane and slab thickness added together and measured down from the finished level. This sum IS the dig depth, and it is the number to get right before anything else.
The takeoff, in order
Each step needs something from the one before it, which is why the order is part of the answer.
Work out the excavated volume and what leaves site
- Needs
- The dig depth from the layer build-up, the plan area including working space, and the ground's fall.
- Produces
- The bank volume to dig, and — after bulking — the LOOSE volume that has to be carted away, which is the larger number and the one the skips are sized on.
Quantify the sub-base
- Needs
- The plan area from the setting out and the COMPACTED sub-base thickness from the layer build-up, not the loose depth it arrives at.
- Produces
- The tonnage, in the COMPACTED state — loose delivered volume is greater, which is why it is ordered by mass rather than by volume.
Result7.92 t (8.73 US tons) of sub-base under the 24 m² (258 sq ft) slab, at a compacted density of 2,200 kg/m³ (137 lb/ft³)
Gravel Base Layer Tonnage CalculatorWork out how many compaction lifts it goes in
- Needs
- The sub-base thickness from the step above and the compaction plant actually available on site, because the plant sets the maximum lift.
- Produces
- The number of passes and lifts. A plate compactor reaches a limited depth, so 150 mm is two lifts rather than one — the step that decides whether the slab settles.
Result2 lifts of 75 mm (3 in), each compacted before the next, for the 150 mm (5.9 in) sub-base
Backfill Compaction Lift Count CalculatorQuantify the damp-proof membrane
- Needs
- The slab area plus the perimeter upstand and the laps.
- Produces
- The membrane area and roll count. The laps and upstand are why this exceeds the slab area, and the commonest shortfall on this line.
Result1 roll of the calculator's 3 × 30 m (10 × 100 ft) sheet: 26.4 m² (284 sq ft) with the lap allowance, before the perimeter upstand is added
Vapor Barrier CalculatorQuantify the formwork
- Needs
- The slab perimeter and its thickness, and the excavation width — the formwork sits OUTSIDE the slab line, which is why the dig is wider than the slab.
- Produces
- The board length, pegs and bracing. Small, and the item most often forgotten because it is temporary.
Result9 boards of 2.4 m for the 20 m (65.6 ft) perimeter (9 of 8 ft on the imperial setting)
Concrete Formwork CalculatorQuantify the reinforcement
- Needs
- The slab area, the mesh or bar specification, and the lap requirement.
- Produces
- The mesh sheets or bar schedule INCLUDING laps, which add materially on a slab this size because two laps cross most of it.
Quantify the concrete
- Needs
- The slab area and thickness, plus the allowance decision below.
- Produces
- The volume — then rounded to how concrete is actually supplied, which is the step that makes this line different from all the others.
Result2.40 m³ (3.14 yd³) of concrete, neat — the allowance and the rounding up to the supplied load come on top
Concrete CalculatorSet the joint spacing
- Needs
- The slab dimensions and thickness from the steps above, and the reinforcement decision, since mesh changes where a slab will tolerate a joint.
- Produces
- Where the joints go, and whether this slab needs any. Deciding this AFTER the pour is how a slab cracks where nobody wanted it to.
Result3.0 m (9.84 ft) maximum spacing by the plain-slab rule at 100 mm (3.9 in), so the 6 m (19.7 ft) and the 4 m (13.1 ft) sides each need at least one joint
Concrete Control Joint Spacing CalculatorQuantify the curing protection
- Needs
- The slab area, the expected weather in the days after the pour, and the mix — a hotter, windier day and a higher-cement mix both raise what the surface loses.
- Produces
- The covering, which costs little and prevents the two commonest slab defects — a dusting surface and curling edges — both of which come from the top drying too early.
Result2 blankets of 1.8 × 7.6 m (6 × 25 ft) for 27.6 m² (297 sq ft) with the overlap
Concrete Curing Blanket Calculator
The figures
| Step | Calculator | Figure |
|---|---|---|
| Quantify the sub-base | Gravel Base Layer Tonnage Calculator | 7.92 t (8.73 US tons) of sub-base under the 24 m² (258 sq ft) slab, at a compacted density of 2,200 kg/m³ (137 lb/ft³) |
| Work out how many compaction lifts it goes in | Backfill Compaction Lift Count Calculator | 2 lifts of 75 mm (3 in), each compacted before the next, for the 150 mm (5.9 in) sub-base |
| Quantify the damp-proof membrane | Vapor Barrier Calculator | 1 roll of the calculator's 3 × 30 m (10 × 100 ft) sheet: 26.4 m² (284 sq ft) with the lap allowance, before the perimeter upstand is added |
| Quantify the formwork | Concrete Formwork Calculator | 9 boards of 2.4 m for the 20 m (65.6 ft) perimeter (9 of 8 ft on the imperial setting) |
| Quantify the concrete | Concrete Calculator | 2.40 m³ (3.14 yd³) of concrete, neat — the allowance and the rounding up to the supplied load come on top |
| Set the joint spacing | Concrete Control Joint Spacing Calculator | 3.0 m (9.84 ft) maximum spacing by the plain-slab rule at 100 mm (3.9 in), so the 6 m (19.7 ft) and the 4 m (13.1 ft) sides each need at least one joint |
| Quantify the curing protection | Concrete Curing Blanket Calculator | 2 blankets of 1.8 × 7.6 m (6 × 25 ft) for 27.6 m² (297 sq ft) with the overlap |
The waste factors, and why these ends of the ranges
| Material | Applied | Why |
|---|---|---|
| Concrete | A modest allowance, then ROUNDED UP to how it is supplied | The asymmetry is the whole point: running short means a cold joint through a slab that was meant to be monolithic, and no amount of speed fixes it once the first load has begun to set. Excess is wasted money; a shortfall is a defect. The rounding to the supplied increment usually dominates the percentage anyway. |
| Sub-base | The upper end, plus an allowance for the dig being over-deep | Excavation is rarely exactly to level, and every millimetre of over-dig is filled with sub-base rather than with nothing. On a machine-dug slab the over-dig is routinely more than the nominal tolerance. |
| Membrane | Area plus laps plus the perimeter upstand | This is not really a waste factor: the laps and upstand are part of the installation, and quantifying from the slab area alone is an error rather than a tight allowance. |
| Mesh reinforcement | Area plus the laps, counted rather than estimated | Laps on a 6 × 4 m slab consume a real fraction of a sheet, and the sheet size against the slab dimensions decides whether it is two sheets or three. Counting the layout beats applying a percentage. |
| Formwork timber | Perimeter plus pegs, with no reuse assumed | It is a small figure and the timber is usually damaged coming off. Assuming reuse on a one-off slab is optimism rather than allowance. |
Where this estimate is most likely to be wrong
Excavated volume and muck away
usually underTwo effects push the same way. The dig is rarely exactly to level and over-dig is normal, and excavated material BULKS when it comes out — so the loose volume leaving site is larger than the bank volume calculated. Estimating skips from the bank figure under-orders them.
Narrow it by: Taking levels on a grid rather than at the corners, and sizing the haulage on the bulked volume rather than the dig.
Sub-base tonnage
usually underIt fills whatever the excavation actually is, not what it was meant to be. Every millimetre of over-dig across 24 square metres is material, and soft spots found during compaction are filled with more of it.
Narrow it by: Checking the formation level across the area before ordering, and allowing for any soft areas found when the first lift is compacted.
Concrete
either wayThe volume is geometry and is usually close. What moves it is the formation level — a slab poured on an over-dug base is thicker than designed across its whole area, which is a surprising amount of extra concrete. The risk is asymmetric: excess is wasted, a shortfall is a cold joint.
Narrow it by: Checking the level across the formation immediately before the pour, not when the sub-base went in.
Membrane
usually underLaps, the perimeter upstand and the taping at penetrations all consume material that the slab area does not account for, and the upstand height on site is usually generous rather than minimal.
Narrow it by: Quantifying from the laid extent including upstand and laps rather than from the slab footprint.
Programme rather than material
usually underThe estimate covers materials. What actually delays a slab is the sequence: sub-base compacted in lifts with a check between them, formwork set and levelled, membrane laid without puncturing it, and a pour that has to be placed and finished in one operation. The concrete is the shortest part of the job.
Narrow it by: Planning the pour day backwards from the delivery, and treating the preparation as the work rather than as the lead-up to it.
Tools this job needs
Frequently asked questions
- How much concrete do I need for a 6 × 4 m garden room base?
- At 100 mm (3.9 in) thick, a 6 × 4 m (19.7 × 13.1 ft) slab is 2.40 m³ (3.14 yd³) of concrete, neat. That is pure geometry. The order adds a modest allowance for a formation that is not perfectly level, then rounds up to the increment the supplier delivers in, because a shortfall leaves a cold joint through a slab meant to be monolithic, while a small surplus is only wasted concrete.
- How much Type 1 sub-base do I need for a garden room base?
- At 150 mm (5.9 in) compacted under the 24 m² (258 sq ft) slab, the base is 3.6 m³ (127 ft³). At 2,200 kg/m³ (137 lb/ft³), about what a compacted Type 1 sub-base weighs in place, that is 7.92 tonnes (8.73 US tons); at 2,100 kg/m³ (131 lb/ft³), the lighter end of what suppliers quote for it compacted, 7.56 tonnes (8.33 US tons). Use your supplier's figure. It goes down in two 75 mm (3 in) lifts, and every millimetre of over-dig takes more.
- How deep should I dig for a garden room concrete base?
- The dig depth is the layer build-up summed downward from the finished slab level: 150 mm (5.9 in) of sub-base, 25 mm (1 in) of sand blinding, the membrane and a 100 mm (3.9 in) slab make 275 mm (10.8 in). Take it from a fixed datum such as the house DPC rather than the ground. On a site falling about 120 mm (4.7 in) across its length, the dig is deeper at the high end.