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Concrete calculator

Concrete calculator for a slab, footing, square or round column or wall. Type the sizes in m, cm, ft or in and see the concrete in m³ or yd³, with extra for waste, and how many bags of ready-mix of 20, 25 or 40 kg or 40, 60 or 80 lb that is. The formula is shown with your own sizes, so you can check it.

How to use the concrete calculator

  1. 1Pick what you pour: slab, footing, column or wall
  2. 2Type the sizes and pick the units
  3. 3Pick the extra for waste and the bag size
  4. The concrete in m³ or yd³, the bags and the formula

What you get

  • The formula with your sizes.Length × width × thickness, or π × r² × height for a round column, worked out step by step
  • Metric and imperial.Sizes in m, cm, ft or in, the volume in m³ or yd³, bags in kg or lb
  • Bags rounded up.A part bag still has to be bought, so the count is always rounded up

What you can do

  • Work out the concrete for a slab, a footing, a square or round column or a wall
  • Mix units: the length in metres and the thickness in centimetres, or feet and inches
  • Add 5 to 10 % for waste, or any other share
  • Count bags of 20, 25 or 40 kg, or 40, 60 or 80 lb
  • Count several footings or columns of the same size at once

Good to know

  • A 10 × 10 ft slab 4 in thick takes 1.23 yd³: 56 bags of 80 lb before the extra for waste
  • One bag of premixed concrete makes about 12 litres per 25 kg, or 0.6 ft³ per 80 lb. Check the bag: products differ
  • For a cubic metre or more, many builders order ready-mixed concrete from a truck instead of mixing bags
  • The volume is for concrete only: gravel under a slab and steel inside are not counted

Questions

How do I work out the concrete for a slab?

Multiply length, width and thickness in the same unit. A slab of 4 × 3 m and 10 cm thick is 4 × 3 × 0.1 = 1.2 m³.

How much extra should I add?

Most builders add 5 to 10 % for spills, uneven ground and what stays in the mixer. Add more when the ground is rough.

How many bags of concrete do I need?

Divide the volume by what one bag makes. At about 11.7 litres per 25 kg bag, 1.2 m³ with 5 % extra takes 108 bags.

How do I work out a round column?

π × radius² × height. A column 30 cm across and 2.5 m high is π × 0.15² × 2.5 = 0.18 m³.

Is what I type sent anywhere?

No. The sums are worked out on your device, not uploaded.

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