Concrete Calculator

How it works

This calculator turns the size of a slab, footing, post hole or column into the volume of concrete it holds, adds a little extra for waste, and then tells you how many bags of premix to carry home. If you are ordering ready-mix from a truck instead, the cubic yard (or cubic metre) figure in the quick answer is the number the plant will ask you for.

The formula

A slab or footing is a box, so its volume is length × width × thickness. A round hole or column is a cylinder: π × diameter² ÷ 4 × depth. When you pour several identical pieces, such as a row of fence posts, the volume of one is multiplied by the count. Every measurement is converted to the same unit before multiplying, which is why you can type the slab size in feet and the thickness in inches without doing any conversion yourself.

A worked example

The page opens with a 10 ft × 10 ft patio slab, 4 in thick. Four inches is one third of a foot, so the slab holds 10 × 10 × 0.333 = 33.33 cubic feet. Adding 10% for waste gives 36.67 cubic feet, which is 1.36 cubic yards (there are 27 cubic feet in a cubic yard). An 80 lb bag of standard mix makes about 0.6 cubic feet, so the slab needs 36.67 ÷ 0.6 = 61.11 bags. You cannot buy part of a bag, so the answer rounds up to 62.

Why waste is added, and how bags are counted

Real pours always use more than the drawing says. Some mix sticks to the wheelbarrow and the mixer, the ground under a slab is never perfectly flat, and forms bow out a little under the weight of wet concrete. The default allowance of 10% covers that for most small jobs; lower it for a very neat pour into rigid forms, or raise it for rough, dug ground. The bag count always rounds up, and the exact figure is shown beside it so you can judge how close you are to the next bag. The planner also lists the count for every common bag size (40 lb bag, 60 lb bag, 80 lb bag), so you can compare how many trips and how much lifting each one means.

Measuring tips

• Measure the inside of the forms, not the outside of the boards.
• Check the depth in several places. If the ground slopes, use the average depth, or split the pour into two simpler shapes and add them together.
• For post holes, measure the hole you actually dug; augers often cut wider than their rated size.

Common mistakes

• Typing the thickness in feet when you meant inches: a 4 ft slab is twelve times more concrete than a 4 in one. The unit picker next to each field shows what the number means.
• Forgetting the gravel base. If the slab sits on compacted gravel, plan that layer separately with the gravel calculator.
• Buying exactly the rounded number with no spare. A bag or two extra on site costs little compared with stopping halfway through a pour.