Concrete slab calculator

Enter the slab dimensions and this gives you cubic yards for a ready-mix order and the bag count if you are mixing it yourself. Thickness is the number that catches people out — it is in inches while everything else is in feet.

Concrete needed 1.36 cu yd

Same volume in cubic feet 36.7 cu ft

Bags of mix 62 bags Rounded up — buy one spare.

Finished slab weight 5,000 lb The slab at its design size, not the extra you order for waste.

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 is sharper than most sites admit. 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%.

Common cases

Questions people ask

How many 80 lb bags of concrete make a cubic yard?

Forty-five. An 80 lb bag yields 0.6 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags. That is 3,600 lb of material, which is why bagged mix stops being sensible around one cubic yard.

How thick should a concrete slab be?

Four inches is standard for patios, walkways and shed bases. Go to five or six inches if a car, truck or heavy equipment will ever sit on it, and reinforce it. Thickness drives volume directly — going from 4 in to 6 in adds 50% to your order.

How much extra concrete should I order?

Ten percent on a prepared, level subgrade. Fifteen percent over gravel or uneven ground. Running out mid-pour leaves a cold joint you cannot undo, so the extra is cheap insurance.

Can I pour a slab in sections?

Yes, if you plan the joints deliberately — form each section and use control joints between them. What you cannot do is stop halfway through a single section and come back tomorrow; the two pours will not bond and the seam becomes a crack.

Last reviewed 2026-07-30