How much concrete do you need?

Multiply length × width × thickness to get the volume in cubic feet (keep every measurement in feet, so a 4-inch slab uses 4 ÷ 12 = 0.33 ft). Divide cubic feet by 27 to get cubic yards. Then add a waste factor — 10% is typical — because spillage, uneven subgrade, and form bulge always eat a little extra. This calculator does all of that for you.

It depends on the bag size, because the number on the bag is weight, not volume. A cubic yard is 27 cubic feet, and one bag yields: an 80 lb bag — 0.60 cu ft, a 60 lb bag — 0.45 cu ft, and a 40 lb bag — 0.30 cu ft. So a cubic yard takes about 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb.

A 10 × 10 ft slab poured 4 inches thick is about 1.23 cubic yards. At 0.60 cu ft per 80 lb bag that’s roughly 56 bags with no waste, or about 62 bags once you add the standard 10% waste factor.

Bagged concrete makes sense for small pours, but mixing dozens of bags by hand is slow and the cold-joint risk goes up. As a rule of thumb, once a pour passes roughly one cubic yard, a ready-mix delivery is usually faster, more consistent, and often cheaper than buying and mixing that many bags. This calculator flags it for you when you cross that line.

General guidance: 4 inches for patios and walkways, 5 to 6 inches for driveways and anything carrying vehicle weight, and 6 inches or more for heavy-duty loads. Always confirm against your local code and project requirements — this is a material estimate, not an engineering specification.

A compactable gravel base of 3 to 4 inches is common under slabs for drainage and a stable subgrade. Tick the gravel-base box above and the calculator gives you the cubic yards of base for the same footprint.