Concrete Slab Calculator (Cubic Yards, Bags and Cost)

Enter the slab’s length, width and thickness and this calculator gives the concrete in cubic yards and cubic metres, the number of bags of premix if you mix it yourself, and the ready-mix cost if you know the price per yard. It adds a waste allowance for uneven subgrade and spillage, which is where most short pours come from. Works in feet and inches or metres and millimetres.

Concrete slab calculator

How it works

Volume = length × width × thickness (thickness converted to feet), times the number of slabs
To order = volume × (1 + waste %)
Cubic yards = cubic feet ÷ 27; m³ = cubic feet × 0.0283168
Bags = volume to order ÷ yield per bag, rounded up (QUIKRETE Concrete Mix: 0.60 cu ft per 80 lb bag, 0.45 per 60 lb, 0.30 per 40 lb)

Worked example

A 12 × 10 ft patio slab, 4 in thick, with 10% extra:

Units: Feet and inches
Length: 12 ft
Width: 10 ft
Thickness: 4 in
Number of slabs this size: 1
Extra for waste and uneven subgrade: 10%
Bag size (if using premix): 80 lb bag (about 0.60 cu ft)
Ready-mix price per cubic yard (optional): $0

The calculator gives:

Concrete to order: 1.63 cu yd (1.25 m³) including 10% extra
Net volume: 40 cu ft = 1.481 cu yd (1.133 m³)
Bags of premix: 74 × 80 lb bags (2.96 tons to carry)
Slab area: 120 sq ft (11.15 m²)

The slab itself is 40 cu ft (1.48 cu yd). With 10% extra you order 1.63 cu yd, or 74 bags of 80 lb premix, which is 2.9 tons of bags to carry and mix: this is about the size where ready-mix starts to make sense, allowing for the plant’s minimum load.

Material and waste notes

Ready-mix plants sell by the yard or quarter yard with a minimum load and a short-load charge; round up and ask.
A 4 in slab on a subgrade that is 1/2 in low uses 12% more concrete than the calculation. Set the forms carefully or raise the waste figure.
Thickened edges, footings under the slab and piers are extra volume; use the footing and post-hole calculators for those.
Rebar or wire mesh and a gravel base are separate materials; see the rebar and gravel calculators.
Bagged concrete for a slab is fine up to about a yard; beyond that, mixing quality and speed suffer and a truck is usually cheaper.

Frequently asked questions

How many bags of concrete for a 10×10 slab 4 inches thick?

33.3 cu ft of concrete, so 56 bags of 80 lb premix before waste, about 62 with 10% extra. That is 1.23 cu yd of ready-mix.

How much concrete for a 12×12 slab?

At 4 in thick, 48 cu ft = 1.78 cu yd (1.96 cu yd with 10% waste). At 6 in, 2.67 cu yd.

How many cubic feet in a yard of concrete?

27. A yard of concrete covers 81 sq ft at 4 in thick, 54 sq ft at 6 in.

How thick should a concrete slab be?

4 in for patios, sidewalks and shed floors, 5 to 6 in for driveways and garage floors, and thicker under heavy equipment. Local code sets the minimum.

How much does a yard of concrete weigh?

About 4,000 lb (2 tons). Bagged concrete weighs a little more per cubic foot of mix than its finished volume suggests, since the water is added.

Sources

Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.