How Much Concrete Do I Need for a Slab?

Updated October 30, 2026

A slab order is a volume, not a square footage. Length and width tell you the footprint. Thickness tells you how much concrete fills that footprint. Ready-mix plants sell that volume in cubic yards, and they usually step the order in quarter yards. If you stop at square feet, you will call the plant with the wrong unit.

Measure the pour, not the whole yard

Measure the inside of the forms. That is the concrete you will actually place. Length and width can be in feet. Thickness for a patio or shed pad is often 4 inches; a driveway may be thicker. Write thickness in inches if that is how you measured it. Mixing feet and inches is fine when each dimension keeps its own unit.

One rectangle is one calculation. An L shape is two rectangles: calculate each leg and add the volumes. Do not average the two widths into a single number. Averaging undercounts the larger leg.

The calculator also has a quantity field. Use it only when you are pouring identical slabs. Two pads that differ in size are two separate runs, not quantity 2.

Turn the measurements into cubic yards

Convert thickness to feet before you multiply. Four inches is 4 ÷ 12, which is 0.333 feet. Then:

• Cubic feet = length (ft) × width (ft) × thickness (ft) × number of identical slabs.
• Cubic yards = cubic feet ÷ 27.

There are 27 cubic feet in a cubic yard. That division is the step people skip when they are used to buying material by the foot.

A 10 by 12 foot slab at 4 inches

One slab, 10 feet long, 12 feet wide, 4 inches thick.

• Thickness in feet: 4 ÷ 12 = 0.333 ft.
• Cubic feet: 10 × 12 × 0.333 = 40 cu ft.
• Cubic yards: 40 ÷ 27 = 1.48 cu yd.

1.48 cubic yards is the exact volume of that box. It is not the number to read to the dispatcher if the base is soft or the forms are a little out of square. You still need a waste factor, and then a round-up to the way the plant takes orders.

Add a waste factor, then round the order

Waste here means extra concrete so the pour can finish. It is not leftover you expect to return. Concrete in a truck is a one-way trip. The calculator accepts 0, 5, 10, 15, or 20 percent.

• 5% — one rectangle, forms that are square, and a base you have compacted and checked with a straightedge.
• 10% — the default for a typical patio or shed pad. On this example, 1.48 × 1.10 = 1.63 cubic yards.
• 15% — an L shape, a thickened edge, or a base that still has low spots.
• 20% — several small pads, or a first pour where you are not confident in the grade.

After waste, the order rounds up to the next 0.25 cubic yard. 1.63 cubic yards becomes 1.75. Plants commonly take quarter-yard steps, and stopping at 1.50 would put you under the volume you just calculated. Coming up a quarter yard short costs more than a small amount left over that you can place in a post hole or a landing pad the same day.

Bags are a different purchase

The calculator estimates ready-mix in cubic yards. It does not count bags. If the job is small enough to mix by hand, read the yield on the bag you will buy. Many 80 lb bags are labeled near 0.60 cubic feet, which is 45 bags per cubic yard (27 ÷ 0.60). Forty cubic feet is about 67 of those bags before waste, and about 74 bags at 10 percent. A 60 lb bag yields less. Use the number printed on the product in your hand. Extra water makes the mix easier to push and usually means less volume from each bag.

What this volume leaves out

The yards above are concrete only. They do not include a gravel base, rebar or mesh, form boards, or a pump. Some plants add a short-load fee below their minimum, often around one yard and sometimes higher. Ask before you treat 1.75 yards as the full price. If you type a price per cubic yard into the calculator, the cost line is that price times the recommended yards. It is not a delivered quote, and it does not know your haul distance.

A local rule may require a thicker slab, reinforcement, or an inspection. This page does not replace that check. It gets you to a quantity you can confirm with the supplier before the truck is scheduled.