Concrete calculator
Enter the size of a slab, footing, or round hole. You get cubic yards for a ready-mix order and bag counts if you plan to mix it yourself.
You need about
cubic yards
- Cubic feet
- 0
- 80 lb bags
- 0
- 60 lb bags
- 0
- 40 lb bags
- 0
Enter numbers greater than zero in every box.
Bag counts assume common yields of 0.60, 0.45, and 0.30 cubic feet. Check the yield printed on your brand's bag.
How the math works
A slab is a box, so its volume is length times width times thickness. Thickness comes in inches, so divide it by 12 first to get feet. That gives cubic feet. A cubic yard holds 27 cubic feet, so divide by 27 to get the number a ready-mix plant will ask for.
A round hole is a cylinder. Halve the diameter to get the radius, convert it to feet, square it, and multiply by pi and the depth. Then add your waste allowance on top.
Bags or a truck?
Mixing by hand gets old fast. Past about a yard, which is roughly 45 of the 80 lb bags, ready-mix is usually cheaper and far less work. Plants often charge a short-load fee on small orders, so ask before you book. For strength, see concrete psi explained. For mixing your own, read the 1-2-3 mix ratio.
General information, not engineering advice. Structural, code, and safety calls belong to a licensed engineer or your local building official.