Concrete Calculator

Enter the size of your slab, patio, footing or post holes to see how much concrete you need in cubic yards or cubic meters, and how many bags of pre-mixed concrete that equals.

How to use this concrete calculator

Pick Slab / footing for rectangular pours (patios, sheds, walkways, footings) or Round column / post hole for fence posts, deck footings and sonotubes.
Enter the dimensions. Slab thickness is in inches (or centimeters) because that is how it is usually specified.
Enter how many identical slabs or holes you are pouring.
Add a waste allowance. 5–10% covers spillage, uneven ground and the concrete left in the mixer.

The formulas

For a rectangular slab:

Volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12 Cubic yards = volume (ft³) ÷ 27

For a round column or post hole:

Volume = π × (diameter ÷ 2)² × depth

The number of bags is the total volume (including waste) divided by the yield of one bag, rounded up to a whole bag.

Bag yields used

Bag size	Approximate yield
40 lb	0.30 ft³
60 lb	0.45 ft³
80 lb	0.60 ft³
20 kg	≈ 9.4 litres
25 kg	≈ 11.7 litres
30 kg	≈ 14.0 litres

The pound yields are the figures commonly printed on US bagged concrete. The metric yields apply the same yield-per-weight ratio, so treat them as approximate and check your bag.

Worked example

A 10 × 10 ft patio poured 4 inches thick:

10 × 10 × 4 ÷ 12 = 33.3 ft³, which is 33.3 ÷ 27 = 1.23 cubic yards.
Adding 10% waste gives 36.7 ft³.
36.7 ÷ 0.60 = 61.1, so you would buy 62 bags of 80 lb concrete.

Practical tips

For larger pours, compare the cost and labor of mixing dozens of bags with ordering ready-mix delivered by truck.
Measure thickness at several points. A slab that averages 4½ in instead of 4 in needs 12.5% more concrete.
The calculator does not subtract the volume taken up by rebar, mesh or a gravel base. Those are usually small enough that your waste allowance covers them.

Frequently asked questions

How many bags of concrete are in a cubic yard?

A cubic yard is 27 cubic feet. At typical label yields that is 45 bags of 80 lb concrete (0.60 ft³ each), 60 bags of 60 lb (0.45 ft³) or 90 bags of 40 lb (0.30 ft³).

How thick should a concrete slab be?

Patios and walkways are commonly poured about 4 inches (10 cm) thick, while driveways and slabs that carry vehicles are often thicker. Your local building code and the load the slab will carry decide the right thickness.

How much extra concrete should I order?

Most people add 5–10% to cover uneven subgrade, spillage and material left in the mixer. Running short in the middle of a pour is worse than having a little left over.

Can I use this for post holes?

Yes. Choose Round column / post hole, enter the hole diameter and depth, and the number of holes. If a post sits in the hole, the concrete needed is slightly less than the result.

Your estimate