Construction & DIY

Concrete Calculator

Size a pour and see how many bags you need.

By TheCalcTools Updated Formula checked and tested

Your numbers

%

Your results

Volume (cu ft)
Volume (cu ft)
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Volume (cu yd)
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Bags needed
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Volume (cu m)
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Concrete Calculator by TheCalcTools

How the Concrete Calculator works

Enter the slab dimensions and thickness.

Volume is length × width × thickness, plus an allowance for waste and uneven subgrade.

Bag counts use the typical yield of premixed concrete.

The formula

Volume (cu ft) = length × width × thickness (ft)1 cubic yard = 27 cubic feet

Worked example

Using the default values shown in the calculator:

Length (ft)10
Width (ft)10
Thickness (in)4
Bag size80 lb bag (0.6 cu ft)
Extra for spillage10 %
  1. 10 × 10 × 4/12 ft = 33.3 cu ft; with 10% extra = 36.7 cu ft (1.36 cu yd).
  2. That is 62 bags of 80 lb.

Result:

Volume (cu ft)36.7
Volume (cu yd)1.36
Bags needed62
Volume (cu m)1.04

Frequently asked questions

What thickness should a patio slab be?

4 inches is typical for patios and walkways. Driveways and garage floors are usually thicker. Check local building codes.

Should I order ready-mix?

Over about 1 cubic yard, ready-mix delivery is usually easier than mixing dozens of bags.

Sources and further reading

Gravel CalculatorOrder the right amount of stone by volume and weight Mulch CalculatorCover your beds to the right depth without over-buying Volume ConverterKitchen, fuel and tank volumes between metric and US/UK units Room Area CalculatorMeasure a room once and get square feet, square meters and perimeter Paint CalculatorBuy the right amount of paint the first time Flooring CalculatorSquare footage, waste allowance and boxes in one step

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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.