Concrete slab calculator

How the number is worked out

Volume is length × width × thickness, with the thickness converted from inches to feet first. A 10 ft × 10 ft slab at 4 in is 10 × 10 × 0.333 = 33.3 cubic feet, which is 1.23 cubic yards. Add 10% for waste and you are ordering 1.36 cubic yards.

Concrete is sold by the cubic yard, and there are 27 cubic feet in a cubic yard — this is where most hand calculations go wrong, because people divide by 3 instead of 27.

Bagged mix or ready-mix?

The crossover arrives earlier than the shelf price suggests. Our 10×10 slab at 4 in works out at 62 × 80 lb bags once waste is included. That is about 5,000 lb of material to carry, open and mix, and at typical big-box pricing it costs more than a short load of ready-mix.

A useful rule: under about 0.5 cubic yards, bag it. Over about 1 cubic yard, call a truck. In between, decide on whether you can get a mixer and a helper.

Two things to check before you order ready-mix: most plants have a minimum load (often 1 cubic yard) and charge a short-load fee below 3–4 yards, and once the truck arrives you have a fixed window to place it. Bagged mix lets you work at your own pace, one section at a time.

What this does not include

• The gravel base. A slab normally sits on 4 in of compacted gravel. That is a separate order, and gravel compacts, so you need more than the raw volume.
• Rebar or mesh. Any slab that will carry weight needs reinforcement.
• Over-excavation. If the subgrade dips, the slab gets thicker there and the volume climbs quickly. On rough ground, use 15% waste rather than 10%.

When to stop trusting this number

The arithmetic above is exact. What it rests on is a set of assumptions, and the answer goes on looking just as confident after they stop holding:

• The bag yields are for general-purpose concrete mix. Fast-setting, high-early-strength and crack-resistant products are different formulations and print their own yield on the sack. If the bag in front of you does not match the figure in the dropdown, the cubic yards here are still right and the bag count is not — divide the cubic feet by the yield on your bag instead.
• 150 lb per cubic foot is normal-weight concrete. Order a lightweight mix and the finished weight shown here is too high. Only use that output for a load check if you know which mix is turning up.
• It is one rectangle at one thickness. A thickened edge, a turned-down perimeter, a haunch where the slab meets a wall, or a slab poured to a drainage fall — none of those are in the volume, and every one of them adds to it. Enter each piece as its own slab and add the results.
• The waste percentage is a guess you make, not a measurement we take. Nothing on this page has seen your subgrade. Over-excavation spread thinly across a large slab is real concrete, and 10% is a convention, not a model of your ground.
• Nothing here says the slab is strong enough. Thickness, reinforcement and what the ground below can carry are design questions that depend on the load and your local code. This page answers how much, not will it hold.
• It does not judge whether a dimension is buildable. The thickness field has no upper limit, because we cannot cite a source for where a slab stops being a slab, and a cap picked without one would refuse a correct entry from someone with an unusual but real job. Type 40 inches and you get the honest volume of a 40 in pour. Whether it is a sensible thing to pour is not a question this page asks.