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Concrete Calculator

Work out how much concrete a slab, footing or round column needs, in cubic metres, cubic yards and bags.

Result

Enter your values and select Calculate to see the result.

Runs in your browser — nothing you enter is sent to a server.

How the Concrete Calculator works

Choose a rectangular slab, footing or wall, or a round column or post hole, and enter its size in metric or US units. The calculator works out the volume, adds a waste allowance for spillage and uneven ground (10% by default), and converts it to cubic metres, cubic yards and bags of premixed concrete.

Formulas

Rectangular

volume = length × width × thickness × (1 + waste)

Round

volume = π × (diameter ÷ 2)² × height × (1 + waste)

Bags

bags = volume in ft³ ÷ bag yield (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³)

Examples

A 3 × 3 m patio, 10 cm thick

0.9 m³, or 0.99 m³ with 10% waste (about 1.29 cubic yards) — 59 bags of 80 lb.

A 10 × 10 ft slab, 4 in thick

About 1.36 cubic yards with 10% waste, or 62 bags of 80 lb.

Frequently asked questions

Bags or ready-mix?

Bags suit small jobs; above about 1 cubic yard (0.75 m³), ready-mix delivered by truck is usually easier and cheaper.

How thick should a slab be?

Around 4 in (10 cm) for patios and paths, and thicker for driveways and heavy loads — check local requirements.

Why add waste?

Forms bulge, ground is uneven and some concrete is always left in the mixer. Running short mid-pour is worse than a little left over.

Information:

Good to know

Estimates for planning. Follow local building codes and the manufacturer's instructions, and use a qualified professional for structural or electrical work.

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