How much concrete do I need for a slab or a footing

Volume is length times width times thickness

Concrete suppliers sell by the cubic metre or the cubic yard. Many Canadian jobs mix feet, inches and metres, so the first job is to put all three measures in the same unit before you multiply.

1. Measure the length and width of the slab.
2. Measure the thickness. Thickness is often given in inches or millimetres, so change it to feet or metres.
3. To change inches to feet, divide by 12.
4. Multiply length, width and thickness. In feet, the answer is in cubic feet.
5. To get cubic yards, divide cubic feet by 27.

Worked example: a small pad (made-up figures)

The pad is 10 feet by 12 feet and 4 inches thick. Four inches is one third of a foot. 10 times 12 times one third is 40 cubic feet. 40 divided by 27 is about 1.48 cubic yards.

In metres, the same pad is about 3.05 by 3.66 by 0.10, which is about 1.13 cubic metres.

The ground is never perfectly flat, and some mix is lost in the forms. Add an extra amount that you choose, and say so on your quote.

Questions to ask the supplier

The volume tells you how much. It does not tell you what to pour. Thickness, mix and reinforcing depend on the load and on local rules. Ask your supplier, and ask your building department or an engineer where the job needs it.

• Is there a minimum order or a short-load charge?
• What does delivery cost, and is it charged by the trip?
• How long can the truck wait on site before extra time is charged?
• Is the price per cubic metre or per cubic yard?

Delivery and short-load charges can change the price more than the mix. Put them on the job's cost list, with Tax A and Tax B added from the official source.

Check your figures with the concrete volume calculator

Type the same figures into the concrete volume calculator if you want a second check. It is free to start, you type every figure yourself, and nothing is sent. Do it on paper first if you like, then use it to catch a slip.

Built in Canada for Canadian rules.