Concrete Slab Calculator
Find how much concrete a slab needs, and how many bags that is. Change any number and the answer follows.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
Multiply length by width by thickness to get the volume, then add a little extra for spills and uneven ground. 10% is common.
Bag counts use 2,400 kg per cubic metre of concrete, about 150 pounds per cubic foot, and round up.
Worked example
A slab 10 ft long and 10 ft wide, 4 in thick, with 10% extra, takes 1.36 yd³ of concrete. That is 69 of the 80 lb bags, or 92 of the 60 lb bags.
Questions
How thick should a slab be?
Many patios and walkways are about 4 inches, or 10 cm. Driveways and sheds often need more. Your local building code has the final say.
How much extra should I order?
Most people add 5 to 10%. Ground is rarely perfectly flat, and running short mid-pour is the bigger problem.
Bags or a truck?
Bags suit small jobs. Above about one cubic yard, many people find a delivered load easier. Ask your supplier for their minimum.
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