Concrete Calculator — Cubic Yards & Bags for Slabs

Enter slab length, width and thickness — get cubic yards, plus how many 80-lb, 60-lb or 40-lb bags to buy.

Concrete Slab Calculator

The formula behind the answer

Concrete is ordered by volume. For a rectangular slab: length × width × thickness ÷ 12 = cubic feet, then cubic feet ÷ 27 = cubic yards. A 10 ft × 10 ft patio at the standard 4-inch thickness works out to 33.3 cu ft, or about 1.23 cubic yards — roughly 55 of the 80-lb bags.

Bags or ready-mix truck?

Below about 1 cubic yard, bagged mix is usually the better deal and the only practical option for a wheelbarrow project. An 80-lb bag yields about 0.6 cu ft of concrete, a 60-lb bag 0.45 cu ft, and a 40-lb bag 0.30 cu ft. Past roughly 1 cubic yard — a two-car driveway pad, say — a ready-mix truck becomes cheaper per yard and saves your back: hand-mixing 55+ bags in one pour is a genuinely hard day.

Don't skip the waste margin

Pros order 5–10% extra to absorb an uneven subgrade, spillage and the slab spot that always runs slightly deeper than planned. The calculator's answer is the exact mathematical volume — add the margin on top.

FAQ

How thick should a concrete slab be?

4 inches is standard for patios and walkways. Driveways that carry cars want 5–6 inches, and a shed or hot-tub pad should be at least 4 inches over compacted gravel.

How many 80-lb bags make 1 cubic yard?

45 bags. One cubic yard is 27 cubic feet and each 80-lb bag yields about 0.6 cubic feet, so 27 ÷ 0.6 = 45.

How long does concrete take to cure?

It sets in 24–48 hours and you can walk on it in a day or two, but full cure to its rated strength takes about 28 days. Keep it damp for the first week in hot weather.

Do I need rebar or wire mesh in a patio slab?

For a 4-inch sidewalk or patio, wire mesh or fiber-mixed concrete is usually enough. Slabs that carry vehicles benefit from #3 or #4 rebar on 2-foot centers, held in the middle of the pour.