Concrete Slab Calculator

Volume = length x width x thickness, with all three in feet, then multiplied by the waste allowance. Divide by 27 for cubic yards. A four inch slab is 0.333 feet thick, not 4.

How thick should a concrete slab be?

Four inches carries foot traffic, patio furniture and a shed. Six inches is the minimum where a car or trailer sits, and heavier vehicles want more plus reinforcement.

How many bags of concrete make a cubic yard?

Forty five bags of 80 pound mix, sixty bags of 60 pound, or ninety bags of 40 pound. That is a lot of mixing, which is why a yard is the usual point to switch to a delivery.

Should I order extra concrete?

Yes. Ten percent is the normal allowance, because a subgrade that dips half an inch across a large slab quietly swallows a surprising amount. Running short mid-pour leaves a cold joint you cannot undo.

Do I need gravel under the slab?

Usually four inches of compacted crushed stone. It drains water away from the underside and gives a uniform base, which is what stops the slab cracking as the ground moves.