Concrete Calculator

Work out how much concrete to order for a slab, patio, shed base or driveway. Get the volume in cubic yards plus the number of premix bags.

How to use the Concrete Calculator

Enter the slab length and width in feet.
Enter the thickness in inches (4 in is typical for patios and walkways).
Add a waste allowance (5–10%).

How it works

Volume (ft³) = length × width × thickness ÷ 12, then ÷ 27 for cubic yards. A waste allowance covers spillage, uneven ground and form flex. Premixed bags yield about 0.60 ft³ (80 lb), 0.45 ft³ (60 lb) and 0.30 ft³ (40 lb). For more than about 1 cubic yard, ready-mix delivery is usually cheaper and easier than bags.

Concrete for a 4-inch slab

Slab size	Cubic yards	80 lb bags
10×10 ft	1.23	56
12×12 ft	1.78	80
10×20 ft	2.47	112
20×20 ft	4.94	223
24×24 ft	7.11	320

Frequently asked questions

How many bags of concrete for a 10×10 slab?

A 10×10 ft slab 4 inches thick needs about 1.23 cubic yards — around 56 bags of 80 lb mix (62 with 10% extra).

How thick should a concrete slab be?

4 inches for patios and walkways; 5–6 inches for driveways and heavier loads.

How much does a yard of concrete cover?

One cubic yard covers about 81 sq ft at 4 inches thick.