Concrete Calculator
Concrete volume = length × width × thickness, and 27 ft³ make 1 yd³. A 12 × 12 ft patio slab 4 in thick is 48 ft³: 1.78 yd³ exact, or 1.96 yd³ with the 10% waste allowance — round up to the ¼ yd³ step your supplier sells. For a price, use the Concrete Cost Calculator.
How it's calculated: Volume = length × width × depth; cubic yards = cubic feet ÷ 27; recommended = exact volume × (1 + waste allowance).
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. BuildNumerics 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 this calculator gives the volume rather than a bag count — divide the volume in cubic feet by the yield printed on your bag. For example, Quikrete Concrete Mix 1101 lists 0.60 ft³ per 80 lb bag, so the 48 ft³ worked example is 80 bags exact, or 88 with the 10% allowance. Other products' yields differ, so check the yield printed on the product you buy.
- Does this include rebar or a gravel base?
- No. This calculator returns concrete volume only. Reinforcement and base material are separate.
- How much concrete for a 10×10 slab 4 inches thick?
- 1.23 yd³ (33.3 ft³) for the exact volume, or 1.36 yd³ with the default 10% waste allowance. Round the order up to the next ¼ yd³ your ready-mix supplier sells.
- How thick should a concrete slab be?
- The calculator's guidance is about 4 in for a typical patio and 6 in for a driveway. Enter the thickness your plans call for — the volume scales directly with it.
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.
Related articles
- Concrete vs Cement: What's the Difference? — Cement is an ingredient; concrete is the finished product made from it. Mixing up the two terms is one of the most common mistakes in construction conversation.
- What Is Concrete Made Of? — Concrete is cement, water, fine aggregate and coarse aggregate — and the ratio between them determines almost everything about the finished material.
- Understanding Concrete Strength and Mix Basics — Concrete strength is usually specified as a compressive strength in PSI (or MPa) — driven mainly by the water-cement ratio, curing, and mix design.
- Exact Quantity vs Recommended Order Quantity — Every BuildNumerics calculator shows two figures on purpose — the exact geometric answer, and a separate, waste-adjusted number you can actually order.
Part of the Concrete calculators.