Concrete Calculator
Length, width, thickness — and you get cubic yards, bag counts, and whether bags or a ready-mix truck is the cheaper call. Decimals welcome: 5.5" is a real spec. Typical thickness: 4" patios & walkways, 5" light driveways, 6" driveways & garages.
Project calculators
The formula, once and for all
Length (ft) × width (ft) × thickness (ft) ÷ 27 = cubic yards. Thickness in inches divided by 12 gives you feet — a 4" slab is 0.333 ft. Contractors shortcut a 4" slab as square footage ÷ 81. Both get you the same number; the calculator above just gets there faster and adds the bag math.
Common questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in feet, then divide by 27 to get cubic yards. A 10×10 slab at 4 inches (0.333 ft) is 10 × 10 × 0.333 ÷ 27 = 1.23 cubic yards. Add 10% for waste and uneven subgrade.
How thick should a concrete slab be?
4 inches for patios, walkways, and shed pads; 5–6 inches for driveways and anything that carries vehicles; 6+ inches with reinforcement for workshops and heavy equipment.
Should I order ready-mix or buy bags?
Bags win on small jobs (under about half a cubic yard). Past one yard — about 45 × 80 lb bags — delivered ready-mix is cheaper once you count your time, and dramatically easier.
How much extra concrete should I order?
10% over the calculated volume. Subgrades are never perfectly level, and coming up short mid-pour creates a cold joint — a permanent weak line in the slab.