Concrete calculator: cubic yards from your slab dimensions

Enter the length and width of the pour in feet and the depth in inches, and this calculator returns the volume in cubic yards — the unit concrete is ordered in — along with the equivalent in cubic feet and the number of pre-mixed bags it would take.

Length (ft)

Width (ft)

Thickness (in)

4 in is standard for a patio or shed base; 5-6 in for a driveway.

Waste allowance (%)

5-10% covers uneven subgrade and spillage.

Concrete needed

1.36yd³

Cubic feet 36.7 ft³80 lb bags 61 bags60 lb bags 81 bags

Volumes assume a rectangular pour of even thickness. Footings, steps and thickened edges should be calculated separately and summed. Waste is applied to the total, which over-states it slightly on large pours.

Ordering ready-mix by the yard is almost always cheaper above about one cubic yard, but bags are the practical choice for a single fence post, a step or a small pad where a truck cannot reach or a minimum order charge would double the cost. The bag counts here use the standard yields printed on the bag: 0.60 cubic feet for an 80 lb bag of concrete mix and 0.45 cubic feet for a 60 lb bag.

Concrete is ordered in quarter-yard increments by most suppliers, and running short mid-pour is the expensive failure — a cold joint forms and the finish suffers. The waste allowance is there to be used: 5% is reasonable for a formed slab on a flat, compacted base, and 10% is the safer default where the subgrade is uneven or the forms are hand-set.

How the calculation works

Volume in cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12, then multiplied by (1 + waste ÷ 100). Cubic yards = cubic feet ÷ 27, because a yard is 3 ft in each direction and 3 × 3 × 3 = 27.

Bags = volume in cubic feet ÷ the yield of one bag. For an 80 lb bag of standard concrete mix that is 0.6 ft³, derived from the 133 lb/ft³ density of cured concrete mix; a 60 lb bag yields 0.45 ft³.

• The pour is a rectangular slab or footing of uniform thickness; steps and thickened edges need to be calculated as separate volumes and added.
• Bag yields follow the manufacturers' published figures for standard 80 lb (0.6 ft³) and 60 lb (0.45 ft³) concrete mix, which assume you mix to the stated water content — over-watering reduces yield and strength.
• Waste is applied to the whole volume, which slightly over-states the allowance for large pours; see the limitation note below.
• No allowance is made for subgrade settling or for a gravel base, which is measured separately.

Sources: Bag yields: manufacturer-published figures for standard 80 lb (0.6 ft³) and 60 lb (0.45 ft³) concrete mix; Cured concrete density: 133 lb/ft³, the figure used to derive bag yield

Worked examples

Questions

How many cubic feet are in a cubic yard?

27. A cubic yard is 3 ft × 3 ft × 3 ft. To go the other way, multiply cubic yards by 27, or divide cubic feet by 27 as this calculator does.

How many 80 lb bags of concrete make a yard?

45 bags at the standard 0.6 ft³ yield, because 27 ÷ 0.6 = 45. For a 60 lb bag the figure is 60 bags, at 0.45 ft³ each.

How thick should a concrete slab be?

Four inches is standard for patios, shed bases and footpaths on a compacted base. Driveways carrying cars are usually 5 to 6 inches with reinforcement, and anything carrying a truck or a heavy vehicle should be specified by an engineer rather than estimated here.

Should I add a waste allowance?

Yes for anything formed by hand. Uneven subgrade, spoil in the forms and spillage all consume concrete. Five percent is usually enough on a flat slab that has been screeded level; ten percent is the safer default.

Does this include the gravel base?

No. The base is a separate volume of compacted crushed stone, typically 4 inches deep, calculated by depth in the same way. Use the gravel calculator for that.

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Written and maintained by Sam Winters. Last updated 2026-11-04. Every figure is computed in your browser; nothing you enter is sent anywhere. Found an error? Tell me — accuracy reports get fixed first.