Concrete Slab Calculator
Patio, driveway, or shed pad — same math. Pick the project, then enter length and width.
This pour is about a cubic yard or more. A ready-mix truck is usually easier than mixing that many bags by hand.
How the numbers are figured
Volume in cubic feet is length × width × thickness, with thickness converted from inches to feet. Divide by 27 to get cubic yards — the unit a ready-mix plant sells. Bag counts divide that volume by the yield on a standard US pre-mix bag, then round up so you buy whole bags.
Yields used here: 40 lb = 0.30 ft³, 50 lb = 0.375 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³. Confirm the number printed on the bag you actually buy.
Common questions
How many 80 lb bags of concrete for a 10×10 slab?
A 10 by 10 foot slab that is 4 inches thick takes about 1.23 cubic yards, or 56 bags of 80 lb mix, before waste. With a 10% waste allowance, plan on 62 bags.
How do you convert a slab to cubic yards?
Multiply length in feet by width in feet by thickness in feet (inches divided by 12). Divide that cubic-foot volume by 27 to get cubic yards.
How much does an 80 lb bag of concrete cover?
A standard 80 lb bag yields about 0.60 cubic feet. A 60 lb bag yields about 0.45 cubic feet, a 50 lb bag about 0.375 cubic feet, and a 40 lb bag about 0.30 cubic feet.
Should I order bags or ready-mix?
Bags are practical under about one cubic yard. Larger slabs are usually cheaper and faster with a ready-mix truck. Always add a little extra so you do not come up short mid-pour.
Patio or driveway — which thickness?
Use 4 inches for a patio, walkway, or light shed pad. Use 6 inches if a car or truck will sit on the slab. The Project menu sets that thickness; you can still type a custom depth.
Fence post calculator · Stairs calculator · Round slab calculator