How to Calculate Concrete for a Slab
To calculate concrete for a slab, multiply length by width by thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. A 10 by 10 foot slab at 4 inches thick is 33.3 cubic feet, about 1.23 cubic yards, before adding 5 to 10 percent for waste.
Worked example
Take a 10 by 10 ft slab at 4 inches thick. Convert the thickness to feet: 4 / 12 = 0.333 ft. Volume = 10 x 10 x 0.333 = 33.3 cubic feet. Divide by 27 to get 1.23 cubic yards.
Add a waste allowance: at 10 percent, order about 1.35 cubic yards. The concrete calculator does all of this and also gives the bag count for each bag size.
Bags or ready-mix?
A 1.23 cubic yard slab is about 56 of the 80 lb bags - a lot to mix by hand. As a rule of thumb, once you pass roughly half a cubic yard, a ready-mix delivery is usually easier and often cheaper than bags.
Most slabs and patios are poured at about 4 inches. Thicker pours and footings need more concrete and sometimes reinforcement.
FAQ
How thick should a concrete slab be?
About 4 inches for patios, walkways, and shed bases. Driveways and areas carrying heavy loads are usually 5 to 6 inches, often with reinforcement.
How many bags of concrete for a 10x10 slab?
A 10x10 ft slab at 4 inches is about 1.23 cubic yards, which is roughly 56 of the 80 lb bags. At that size, ready-mix is usually the better option.