Concrete calculator
Enter the size of a slab, a footing or round columns to get the volume of concrete and the number of premix bags.
Concrete needed
How it works
slab = length × width × thicknesscolumn = π × (diameter ÷ 2)² × heightThickness and diameter are entered in centimetres (or inches); the waste percentage is added at the end. 1 m³ = 1.308 yd³.
Worked example
- 4 m × 3 m × 0.10 m = 1.2 m³
- + 10% waste = 1.32 m³
- = 1.73 yd³, or 115 bags of 25 kg
Frequently asked questions
How much extra concrete should I order?
Add 5–10% for spillage, uneven ground and forms that bow. The waste field does this for you.
How many bags make a cubic metre?
It depends on the bag. A 25 kg bag of premix gives about 0.011 m³, so roughly 90 bags per m³; a US 80 lb bag gives about 0.6 ft³ (45 bags per cubic yard). Check the yield printed on the bag.
When should I order ready-mix instead of bags?
Above about 1 m³ (1.3 yd³) ready-mix from a truck is usually cheaper and much less work than mixing bags by hand.