How to work out how much concrete you need
- Choose Feet and inches or Metres and cm. Length and width go in feet or metres, thickness in inches or centimetres.
- Enter the Length, Width and Thickness of the slab, and how much Extra for waste to allow.
- Read the volume in cubic yards or cubic metres, and how many 80, 60 or 40 lb bags of premix that is.
How the volume is worked out
Volume is length × width × thickness, with all three in the same unit. The calculator turns your inches or centimetres into feet or metres for you, adds the waste allowance, then converts the total to every unit.
| Slab | Cubic feet | Cubic yards | Cubic metres | 80 lb bags |
|---|---|---|---|---|
| 10 × 10 ft, 4 in, no extra | 33.3 | 1.23 | 0.944 | 56 |
| 10 × 10 ft, 4 in, 10% extra | 36.7 | 1.36 | 1.038 | 62 |
| 3 × 3 m, 10 cm, 10% extra | 35 | 1.29 | 0.99 | 59 |
Bag counts are rounded up to whole bags. A little thickness makes a big difference: going from 4 to 5 inches adds a quarter to the volume.
Bags or ready-mix
The bag counts assume typical yields for common US premixes: about 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb. Brands vary, so read the yield on the bag you plan to buy and divide the cubic feet by it if it’s different. Bags sold in kilograms also print their yield, usually in litres or cubic metres.
Bags suit small jobs like fence posts, steps and small pads. Past a cubic yard or so you are mixing and carrying dozens of heavy bags, so get a price for ready-mixed concrete delivered by truck too. Suppliers sell it by the cubic yard or cubic metre, which is the number at the top of the result.