Concrete calculator
Concrete and bags needed for a slab.
About this calculator
Enter the slab’s length and width in feet and its thickness in inches. A waste allowance covers spillage and uneven ground.
Bag yields vary by product, so check the label before buying, and get professional advice for anything structural.
How it works
Volume is length × width × thickness, plus your waste allowance. There are 27 cubic feet in a cubic yard.
Worked example
- Length: 10 ft
- Width: 10 ft
- Thickness: 4 in
- Extra for waste: 10%
Concrete needed: 1.36 yd³ (36.7 ft³)
The story behind it
Where the idea came from: Joseph Aspdin, an English bricklayer, who patented Portland cement in 1824.
Romans were building with a form of concrete 2,000 years ago, and the Pantheon's dome in Rome still stands. Modern concrete begins with Aspdin's Portland cement. The maths on site has stayed simple: length times width times thickness, plus some extra for waste. This calculator does it for you, so you don't over-order or run short.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
Meet the founder