Concrete Calculator

Work out how much concrete you need for a slab, footing, column, or steps, with extra for waste. The concrete calculator gives the volume and the number of premix bags.

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Units

Concrete needed
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How to use the Concrete Calculator

  1. Choose your Units and the Shape.
  2. Enter the dimensions, How many pieces, the Extra for waste, and your Bag size.
  3. Press Calculate. The total volume appears with the volume before waste, the other unit, the bag count, and the weight.

How it works

slab or footing = length × width × thickness
round column = π × (diameter ÷ 2)² × height
steps = width × run × rise × n(n + 1) ÷ 2 + platform
total = volume × quantity × (1 + waste)
bags = total × 2,400 kg/m³ ÷ bag weight, rounded up

Thicknesses and step sizes are in cm or inches and converted to metres or feet. 1 cubic yard = 0.7646 m³.

Examples

  • A 3 m × 4 m slab, 10 cm thick: 1.2 m³ before waste, 1.32 m³ with 10% extra, or 127 bags of 25 kg.
  • A round column 30 cm across and 3 m tall: 0.212 m³.
  • 4 steps, 1.2 m wide, with 18 cm rises and 28 cm runs: 0.605 m³.

Bags or ready-mix?

Premix bags suit small jobs such as post holes and pads. Above about 1 m³ (1.3 cubic yards), mixing that many bags by hand is slow and uneven, and a ready-mix truck is usually cheaper per cubic metre.

Limitations

  • Bag counts assume concrete weighs 2,400 kg/m³. Real bag yields vary by product; check the label.
  • Uneven ground and forms that bow can use more concrete than planned, which is what the waste allowance covers.

Frequently asked questions

How much concrete do I need for a slab?

Multiply length × width × thickness in metres, then add 5% to 10% for waste.

How many bags of concrete are in a cubic metre?

About 96 bags of 25 kg, or 60 bags of 40 kg, at 2,400 kg/m³.

How thick should a concrete slab be?

Around 10 cm (4 in) for patios and paths, and 15 cm (6 in) or more for driveways.

Often used together with the Concrete Calculator.