PreConstruct AIA slab drawn at 125 mm instead of 100 mm takes a quarter more concrete, and nothing about the formwork tells you which one you are looking at.
Thickness is the number a slab takeoff turns on, and it is the one most often carried over from the last job instead of read off this one. This post gives the conversions, the concrete each common thickness takes, where the number on a drawing comes from, and a worked driveway where the detail that matters is not on the plan.
Where the thickness comes from
A slab thickness is not a rule of thumb. It is a number on a drawing: the structural set for a structural slab or a footing, the architectural set or a slab detail for a floor on grade, a municipal standard for public sidewalks and aprons.
There is also a code floor under it for houses and small buildings. Part 9 of the Ontario Building Code says a concrete slab on ground shall be not less than 75 mm thick, not counting a topping (article 9.16.4.3 in the 2012 edition). That is a minimum, not a design, and the 2024 Ontario edition has been in force since 1 January 2025, so check the edition your permit names before you quote it. Outside Ontario, the National Building Code and your province's adoption of it set the same kind of minimum; read the one that applies.
Footings are a different case. A footing's width and depth come from the engineer, the soil and the load on it, and they belong on the structural drawings. There is no thickness safe to bring from another job.
Millimetres, inches and the concrete each takes
The conversions are exact enough to order from: 25.4 mm to the inch, so the common metric thicknesses are close to, but not the same as, the round imperial ones.
| Thickness | In inches | Concrete per m² | Concrete per 100 ft² |
|---|---|---|---|
| 75 mm | 2.95 in | 0.075 m³ | 0.91 yd³ |
| 100 mm | 3.94 in | 0.100 m³ | 1.22 yd³ |
| 125 mm | 4.92 in | 0.125 m³ | 1.52 yd³ |
| 150 mm | 5.91 in | 0.150 m³ | 1.82 yd³ |
| 200 mm | 7.87 in | 0.200 m³ | 2.43 yd³ |
The metric column is the easy one: thickness in metres times area in square metres is the volume. For the imperial column, 100 ft² at a thickness of t inches is 100 × t ÷ 12 ÷ 27 cubic yards. A drawing that says 4 in rather than 100 mm takes 1.23 yd³ per 100 ft², a hair more than the 100 mm row, and on a large slab the difference between the two is worth checking against the unit the drawing actually used.
What you will commonly see on drawings, as a sense check and never as a substitute for the sheet:
- interior floors on grade, basements included: 75 to 100 mm (3 to 4 in);
- garage floors and driveways: 100 to 150 mm (4 to 6 in), thicker where a vehicle is heavier or a post lift is planned;
- sidewalks and exterior flatwork: around 100 mm (4 in), or whatever the municipal standard says;
- structural slabs and footings: the engineer's number, with no typical value worth carrying.
Two things that change the pour
A thickened edge. Garage slabs and driveways are often detailed with the perimeter poured deeper than the field, sometimes turned down into a small footing where a wall or a wheel load sits. The plan may show only the field thickness; the edge is in a section. Miss it and you order short.
What the thickness is measured to. A note normally means top of concrete to the underside of the slab. Reinforcing mesh or rebar sits inside that thickness at its own depth. Confusing "to the top of the mesh" with "to the subgrade" does not change the volume, but it changes the screed depth, and that shows up as a thin spot.
Worked example: a driveway with a thickened edge
The drawing gives a driveway 15.0 m long by 3.6 m wide, field thickness 100 mm. A section shows a thickened edge all round, 300 mm wide, 200 mm deep in total, so 100 mm deeper than the field.
- Field: 15.0 × 3.6 = 54.0 m², × 0.100 m = 5.40 m³.
- Edge strip area: the perimeter is 2 × (15.0 + 3.6) = 37.2 m. A 0.3 m strip all round is 37.2 × 0.3 = 11.16 m², less the four corners counted twice (4 × 0.3 × 0.3 = 0.36 m²), so 10.80 m².
- Extra depth at the edge: 0.200 − 0.100 = 0.100 m, so 10.80 × 0.100 = 1.08 m³ on top of the field.
- Total before waste: 5.40 + 1.08 = 6.48 m³.
- With your own waste allowance, say 5 per cent for subgrade that is not flat and what stays in the truck: 6.48 × 1.05 = 6.80 m³. Order 7.0 m³, or 9 yd³ from a plant that sells in yards (6.80 × 1.308 = 8.89).
The thickened edge is about a sixth of the pour. Taken off the plan's 100 mm alone, the order comes up over a cubic metre short, which on a driveway is the difference between finishing and waiting for a second truck.
A checklist for the next slab
- Read the thickness off this job's drawings: the plan, then every section that cuts the slab.
- Look for a thickened edge, a turned-down footing or a depressed area in the sections.
- Note the unit the drawing uses, convert once, and write both figures down.
- Treat the code minimum as a floor, never as the design, and check the edition.
- Add your waste allowance before rounding up to what the plant sells.
- For footings and structural slabs, use the engineer's numbers and nothing else.
Thickness is half the concrete order, and the other half is finding every place on the set where it changes. PreConstruct AI reads the concrete notes on a set on request and shows each thickness it finds with its words and the sheet it came from, so you are checking a citation rather than a memory. Your first bid is free, on a set of up to 75 sheets.