Concrete Calculator

Enter your slab or footing size. Get cubic yards and bags of mix.

100% in your browser - no upload, no sign-up.

How to use this concrete calculator

1. Pick your pour: slab, footing or round column.
2. Enter the dimensions - length, width and thickness for a slab or footing, or diameter and height for a round column.
3. Read the volume in cubic yards and the bag counts for 40, 60 and 80 lb bags. Add a waste allowance if your subgrade is uneven or you expect spillage.

How concrete volume is calculated

For a slab or footing the volume is length × width × thickness, with the thickness converted from inches to feet. A round column is π × radius² × height. That gives cubic feet, which is divided by 27 to get cubic yards, since 27 cubic feet make one cubic yard. Bag counts come from each mix's published yield: an 80 lb bag of pre-mixed concrete yields about 0.60 ft³, a 60 lb bag 0.45 ft³, and a 40 lb bag 0.30 ft³. Bags are always rounded up to whole bags.

Frequently asked questions

How much concrete do I need for a 10x10 slab?

A 10 ft by 10 ft slab poured 4 inches thick works out to about 1.23 cubic yards, or 33.3 cubic feet. That's roughly 56 of the 80 lb bags - enough that ordering ready-mix by the yard is usually cheaper and far less work. Change the thickness above to match your pour.

How many bags of concrete are in a cubic yard?

One cubic yard is 27 cubic feet. Divided by each bag's yield, that's 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags. It's why anything past roughly half a yard is usually cheaper delivered as ready-mix.

Should I order concrete by the yard or buy bags?

Bags make sense for small jobs - footings, posts, or a slab under about a third of a yard. Beyond roughly one cubic yard the bags get heavy, slow to mix and often pricier than a ready-mix delivery, so ordering by the yard usually wins. Add 5–10% for waste either way.