Concrete & Masonry
Concrete Slab Calculator
Enter slab dimensions in metric or imperial — Canadian ready-mix is ordered in cubic metres, and this calculator converts either way and adds a wastage allowance.
Formula tested · Local units · No sign-up
Project inputs
Enter measurements
Use your preferred units. Results update automatically.
Show the calculation methodFormula, conversions, rounding, and assumptions+
Volume = length × width × thickness, converted to a common unit first. Canada is officially metric — ready-mix plants sell by the cubic metre — but many DIY measurements are still taken in feet and inches, so the calculator accepts both and does the conversion for you.
The total is rounded up to the next quarter cubic metre to match ordering practice.
Real-world example
Worked example: 4 m × 3 m garage pad, 10 cm thick
- Convert thickness: 10 cm = 0.1 m.
- Volume: 4 × 3 × 0.1 = 1.2 m³.
- Add 10% wastage: 1.32 m³.
- Round up to the ordering increment: 1.5 m³.
Order 1.5 m³. Enter your local ready-mix quote and combined GST/PST or HST rate for a cost estimate — rates differ by province.
Before you start
How to measure
- Measure length and width at the formwork; the calculator accepts metres, centimetres, feet or inches per field.
- If you measured in feet and inches, you can keep those units — the tool converts to cubic metres for ordering.
- Measure thickness at several points and use the largest, especially over frost-susceptible ground where the base may be uneven.
Local guidance
Notes for Canada
- Canadian ready-mix is sold in cubic metres even though lumber and many site measurements remain imperial — double-check which units your contractor's numbers are in.
- Freeze–thaw exposure matters in most of Canada: air-entrained concrete is standard for exterior slabs. Tell your supplier the slab is outdoors.
- Sales tax is 5% federal GST plus provincial tax or a combined HST depending on the province — enter your combined rate.
Quick reference
Slab thickness quick reference (typical nominal values)
| Application | Common nominal thickness |
|---|---|
| Patio / walkway | 100 mm (4 in) |
| Garage pad | 100–150 mm (4–6 in) |
| Driveway | 125–150 mm (5–6 in) |
| Shed base | 100 mm (4 in) |
Nominal values for planning only — frost depth, soil and local code govern.
Good to know
Common mistakes to avoid
- Mixing unit systems mid-calculation — measuring length in metres but thickness in inches without telling the calculator.
- Ordering exterior concrete without air entrainment in freeze–thaw regions.
- Skipping the wastage allowance over uneven granular base.
- Assuming the tax rate — GST/HST/PST combinations differ by province.
Need help?
Frequently asked questions
Why does my supplier quote in cubic metres when I measured in feet?
Canadian ready-mix converted to metric decades ago. Measure however you like — this calculator converts feet-and-inch measurements into the cubic metres you'll actually order.
How many cubic metres for a 12 ft × 12 ft pad at 4 inches?
12 × 12 × (4/12) = 48 cubic feet ≈ 1.36 m³. With 10% wastage that's about 1.5 m³.
Do I need air-entrained concrete?
For exterior slabs in most of Canada, yes — air entrainment protects against freeze–thaw damage. Your supplier will specify the right mix if you tell them the application.
Keep planning
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- Canada
- Spotted an error?
- Report a correction
Methodology
- Volume is computed as length × width × thickness × number of slabs, converted internally to SI units (metres) before any arithmetic to avoid unit drift.
- The wastage allowance is applied to the exact volume, then the total is rounded UP to the next 0.25 of the regional ordering unit, because ready-mix suppliers typically sell in quarter-unit increments.
- The cost estimate simply multiplies the suggested order quantity by the price you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Unit definitions: 1 ft = 0.3048 m (exact); ready-mix ordering unit is the cubic metre.
- Ordering increments: Quarter-cubic-metre increments are typical; confirm with your plant.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not size reinforcement, check ground conditions, or replace professional structural advice. Confirm quantities and mix specification with your supplier before ordering.