Concrete Calculator
How much concrete a rectangular slab or footing needs, in cubic yards or cubic metres.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Volume = Length × Width × Depth
Cubic yards = Cubic feet ÷ 27 (1 yd³ = 27 ft³)
Run the calculation to see each step with your own numbers.
A 12 ft × 12 ft patio slab, 4 in thick, with the default 10% waste allowance.
Volume = 12 ft × 12 ft × (4 in ÷ 12) = 48 ft³
48 ft³ ÷ 27 = 1.78 yd³ exact
1.78 yd³ × 1.10 = 1.96 yd³ recommended (round up to the next ¼-yd³ your supplier sells)
- Uniform thickness across the whole area — no thickened edges or haunches.
- Rectangular plan. Use a separate calculator for L-shaped or circular pours.
- The waste allowance covers spillage and subgrade variation, not formwork error.
- Excludes reinforcement, vapour barrier and any gravel base.
- Why show an exact and a recommended amount?
- The exact amount is the pure geometry of your slab. The recommended amount adds a waste allowance for spillage and an uneven subgrade so you don't come up short mid-pour. BuildFactor never hides the waste — you can see the percentage and change it.
- How many 80 lb bags is that?
- Bag yield varies by product, so BuildFactor does not convert to a bag count here until a manufacturer-cited yield is validated. For pours above roughly 1 yd³, ready-mix delivery is usually more practical than bags.
- Does this include rebar or a gravel base?
- No. This calculator returns concrete volume only. Reinforcement and base material are separate.
Volume formula
Standard solid geometry — Volume = Length × Width × Depth for a rectangular slab or footing.
Unit conversion (yd³ ↔ ft³ ↔ m³)
NIST Special Publication 811 (2008), Appendix B. 1 ft = 0.3048 m exactly, so 1 yd³ = 27 ft³ = 0.764554857984 m³.
Waste allowance default
The 10% default is a common rule-of-thumb allowance for spillage, over-excavation and an uneven subgrade. It is always editable, and the exact (un-adjusted) figure is shown alongside it.
About the Concrete Calculator
The Concrete Calculator works out how much concrete a rectangular slab, footing or pad needs, in cubic yards or cubic metres. Enter the length, width and thickness and it returns the exact volume plus a recommended volume with your waste allowance added, so you don't come up short mid-pour. For round column forms use the Sonotube Concrete Calculator; for a price rather than a volume use the Concrete Cost Calculator.
What you enter
- Length
- Width
- Depth / thickness — A 4 in slab is typical for a patio; 6 in for a driveway.
- Waste allowance — Default 10%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Concrete volume — Ready-mix is usually sold in ¼-yd³ increments — round the recommended figure up to the next increment your supplier offers.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Cubic Yard Calculator — Convert a length, width and depth into cubic yards, cubic feet or cubic metres.
- Concrete Cost Calculator — Estimated material cost of a concrete pour from its volume and a unit price.
- Concrete Weight Calculator — Weight of a concrete volume, for transport and load planning.
- Sonotube Concrete Calculator — Concrete volume for round column forms (Sonotube-style tubes).
- Concrete Calculator for Fence Posts — Concrete volume to set a run of fence posts, allowing for the posts displacing concrete in each hole.
Part of the Concrete calculators.