Concrete calculator for slabs, patios and pads

Concrete is sold by volume: cubic yards from a ready-mix plant (cubic metres outside the US), or bags whose yield is printed on the side. One cubic yard is 27 cubic feet, enough for 81 sq ft of slab at 4 in thick. If you say “cement”, this is still the calculator you want — cement is the powder that binds concrete together, and you buy it already blended with sand and gravel as concrete mix. Cement vs concrete explained.

How to calculate concrete for a slab

Multiply the slab’s area by its thickness to get the volume, convert to cubic yards, then add a little for spillage, form bulge and a base that is a bit low in places. For bags, divide the total by the yield of one bag.

cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
cubic yards = cubic feet ÷ 27
round slab area = 3.1416 × (diameter ÷ 2)²
bags = cubic feet × (1 + extra) ÷ bag yield (cu ft)

A quick check you can do in your head: square feet covered by one cubic yard = 324 ÷ thickness in inches. At 4 in, one yard covers 81 sq ft; at 6 in, 54 sq ft.

Worked examples people search for

• How much concrete for a 10×10 slab — the classic patio, and whether bags still make sense.
• How much concrete for a 12×12 slab — almost exactly two yards with the extra.
• How much concrete for a 20×20 slab — a double-garage-sized pad at 4 and 6 inches.
• How many yards of concrete for a 24×24 slab — the common detached garage.
• How much concrete for a 30×40 slab — shops and pole barns, and planning two trucks.
• How many bags of concrete for a 4×4 slab — AC pads and grill pads in 60 and 80 lb bags.
• How many bags of concrete for an 8×8 slab — hot tubs and small shed pads.
• How much concrete for a 10×12 shed — a shed floor, with or without a thickened edge.

Thickness: what the calculator can and can’t tell you

Quikrete’s data sheet says its standard Concrete Mix is for pours 2 in thick or more. Patios, walks and small pads are commonly poured around 4 in. For driveways, garage floors and any slab that carries a building, the thickness — along with any reinforcement and thickened edges — comes from your plans and your local building code, so enter what they specify. The calculator only works out how much concrete that takes.

For driveways and other exterior slabs in freeze-thaw climates, order air-entrained ready-mix: NRMCA’s guide to scaling recommends about 6% air for slabs exposed to freezing and de-icing salt. Standard bagged mixes such as Quikrete’s Concrete Mix don’t list air entrainment on their data sheets, which is one more reason a driveway is a truck job.

Measure what your forms actually give you. A form made from 2×4s set on edge is 3½ in deep, not 4, because a 2×4 is really 1½ × 3½ in. Measure the depth from the top of the form down to the compacted base in several places; a base that dips half an inch in the middle can add 10% or more to the pour.

Do I need rebar or mesh?

Reinforcement doesn’t stop concrete cracking — all slabs crack a little as they shrink and move. What steel and fibers do is hold cracks tight so the pieces stay level, and control joints decide where the cracks go.

• Welded wire mesh — a light steel grid set in the middle of a slab. Often used in patios, walks and shed floors. It has to sit up inside the concrete on chairs to do anything. See the wire mesh calculator.
• Rebar — heavier bars tied in a grid, used where plans call for more strength, such as driveways, garage floors and thickened edges. See the rebar calculator.
• Fibers — synthetic or steel fibers mixed into the concrete (bagged “crack-resistant” mixes, or added by the ready-mix plant). They reduce small shrinkage cracks but don’t replace steel where steel is specified.

Many small 4 in pads are poured with mesh or a fiber mix, and some with neither. For anything carrying a structure, a hot tub or vehicles, ask your building department — and for a shed or spa, the maker’s installation guide — what reinforcement and thickness they require. The calculator counts concrete either way; the steel adds nothing to the order.

How much extra to add

NRMCA’s guide to ordering ready-mix suggests ordering 4–10% more than the volume worked out from the plan dimensions. The calculator starts at 10%:

• 5% — a small slab with straight, braced forms on a level, compacted base.
• 10% — the usual allowance. Covers spillage, a little form bulge and a base that is slightly low.
• 15% or more — uneven or hand-dug ground, or a pour you can’t easily top up if you run short.

Running short is much worse than having a little left over. A slab poured in two parts with a gap between them leaves a weak cold joint, and a second ready-mix delivery can cost far more than the extra concrete would have.

Bags or a truck?

An 80 lb bag makes about 0.6 cu ft, so a cubic yard is 45 bags and about 3,600 lb of dry mix to lift. Up to about a yard, bags and a rented mixer are manageable for two people. Past that, a ready-mix truck is usually less work and gives a more even slab; the bags vs ready-mix calculator compares the two, and the bag yield chart lists every bag size.

Before you pour

• Compact the base and set the forms level (or with the slope you want for drainage). The gravel base calculator sizes the stone under the slab.
• Have enough people and tools ready before the first bag is mixed or the truck arrives. Concrete waits for no one on a hot day.
• Plan control joints. Quikrete recommends cutting them about 1 in deep every 6–8 ft.
• Keep the slab damp, or use a cure-and-seal, for the first several days. See how long concrete takes to cure.