Concrete Calculator
Concrete volume is length × width × thickness in feet, divided by 27 for cubic yards. Pick a slab or round column, enter the dimensions, and get cubic yards plus the bags you need.
How to calculate concrete volume
For a slab or footing, multiply length × width × thickness, all in feet, then divide by 27 to get cubic yards. A 4-inch slab uses a thickness of 4 ÷ 12 = 0.33 ft. For a round column, the calculator uses the cylinder formula instead. Total several identical pours with the quantity field.
Bags vs. ready-mix
Bagged concrete is convenient for posts and small pads, but it adds up fast — a single cubic yard is 45 eighty-pound bags. For anything over about 1 cubic yard, ordering ready-mix is cheaper and faster. The calculator shows both the volume (for ready-mix) and the bag count (for small jobs).
Don’t forget reinforcement
Most slabs and footings need steel. Size your bars and spacing with the rebar calculator, and use the cubic yard calculator for the gravel sub-base.
Frequently asked
How many cubic yards of concrete do I need?
Multiply length × width × thickness in feet, then divide by 27. A 10×10 ft slab at 4 inches thick is about 1.23 cubic yards.
How many bags of concrete make a cubic yard?
It takes 45 eighty-pound bags, 60 sixty-pound bags, or 90 forty-pound bags to make one cubic yard. Bagged concrete only makes sense for small pours — order ready-mix above about 1 cubic yard.
How do I calculate concrete for a round column?
Use the volume of a cylinder: pi × radius² × height. Switch the pour type to round column and enter the diameter and height.