How many bags of concrete do I need?

Divide the volume in cubic feet by the bag's yield: roughly 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag, and 0.60 for an 80 lb bag. Always add about 10% for waste and spillage.

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Concrete Calculator

Work out the volume of a concrete slab or footing, in cubic yards and cubic metres, and estimate how many bags of pre-mix you will need.

Concrete volume

1.23 yd³

Cubic metres—

Cubic feet—

60 lb bags (+10%)—

80 lb bags (+10%)—

How it works

Concrete is priced and poured by volume. Multiply length by width by thickness (all in the same units) to get the volume, then convert to whatever you need — cubic yards for US ready-mix, cubic metres elsewhere, or cubic feet to count bags. Always order a little extra for spillage and uneven ground.

volume = length × width × thickness × quantity
1 yd³ = 27 ft³ · 1 m³ ≈ 35.31 ft³
bags = ft³ ÷ bag yield (0.30 / 0.45 / 0.60 ft³ for 40 / 60 / 80 lb)

Worked example

A 10 ft × 10 ft slab at 4 in thick is 100 ft² × 0.333 ft = 33.3 ft³, which is about 1.23 cubic yards. With a 10% allowance that is roughly 82 sixty-pound bags — enough that ready-mix delivery is usually cheaper at this size.

Frequently asked questions

How do I calculate concrete for a slab?

Length × width × thickness in matching units gives the volume; convert to cubic yards or metres to order.

How many bags do I need?

Divide cubic feet by the bag yield (about 0.60 ft³ for an 80 lb bag) and add ~10% for waste.