Concrete Calculator
Find the cubic yards of concrete for a slab from length, width and thickness, plus how many 60 lb or 80 lb bags it takes.
How it works
cubic yards = length × width × (thickness ÷ 12) ÷ 27 · then add ~10% for waste
Length and width are in feet and thickness in inches, so thickness is divided by 12 to get feet; multiplying the three gives cubic feet, and dividing by 27 converts to cubic yards (27 cubic feet make a yard). Bag counts come from each bag’s yield — a 60 lb bag makes about 0.45 cu ft and an 80 lb bag about 0.60 cu ft. Always order a little extra, because subgrade is never perfectly flat.
Worked example
A 10 × 10 ft slab at 4 in: 10 × 10 × (4 ÷ 12) = 33.3 cu ft, ÷ 27 = 1.23 cubic yards. That is about 75 sixty-pound bags or 56 eighty-pound bags — well past the point where ordering a ready-mix truck is easier and cheaper.
Frequently asked questions
How much extra concrete should I order?
Add about 10%. Subgrade dips, spillage and a slightly over-dug form all eat into the pour, and running short mid-pour risks a cold joint. Ready-mix suppliers expect you to round up to the quarter or half yard anyway.
When are bags cheaper than ready-mix?
Bags win for footings, posts and pads under roughly ½ cubic yard. Past that, mixing 30-plus bags by hand is exhausting and inconsistent — a short-load or ready-mix truck is worth it.
What thickness does a slab need?
Four inches is standard for patios, walkways and shed floors; go to 5–6 in for driveways and anything carrying vehicles. Local code and soil conditions set the real minimum, so confirm before you pour.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.