Concrete slab calculator: square feet and cubic yards

Concrete is ordered in cubic yards. To convert: (square feet × thickness in feet) ÷ 27. A 200 sq ft slab at 4 inches (0.33 ft) thick needs (200 × 0.33) ÷ 27 = 2.4 cubic yards. Round up to 2.5 yards when ordering.

Standard thicknesses: 4 inches for sidewalks and patios, 5–6 inches for driveways, 6 inches or more for vehicle pads with heavy loads.

Concrete is unforgiving — when the truck leaves, that’s what you’ve got. Always order 5–10% extra to account for grade variations, formwork bulge, and overpour.

Most ready-mix companies have a minimum order (usually 1 yard) and charge a “short load” fee for orders under 3–4 yards. For small slabs, bagged concrete may actually be cheaper.

Concrete is poured at a thickness, so the calculation needs three dimensions, not two. Concrete is priced and ordered by cubic yards, but slabs are usually planned by square feet.

Conversion formula: cubic yards = (length × width × thickness in inches) ÷ 12 ÷ 27.

Standard slab thicknesses: patio 4 inches, driveway 4-6 inches, garage floor 4-6 inches, sidewalk 4 inches, RV pad 6-8 inches, foundation footings 8-12 inches.

Worked example: 20 × 30 ft patio at 4 inches = (20 × 30 × 4) ÷ 12 ÷ 27 = 7.4 cubic yards. Always order at least 10% extra for irregularity in the subgrade.

Standard ready-mix delivery is in 1-yard increments, minimum 1-3 yards. Below that, use bagged concrete (60 lb bag = 0.45 cu ft; an 80 lb bag = 0.6 cu ft).

Concrete projects typically follow standard dimensions. Knowing these helps with rough budgeting:

Garage floor (2-car): 400 sq ft (20×20). At 4 inches: 4.94 cu yd. At 6 inches: 7.4 cu yd.

Driveway (single-car length): 18 × 10 ft = 180 sq ft. At 4 inches: 2.22 cu yd. At 6 inches: 3.33 cu yd.

Driveway (two-car width, 20 ft length): 20 × 20 = 400 sq ft. At 4 inches: 4.94 cu yd.

Standard patio: 12 × 12 = 144 sq ft. At 4 inches: 1.78 cu yd.

Large entertaining patio: 20 × 30 = 600 sq ft. At 4 inches: 7.4 cu yd.

Sidewalk: 50 linear ft × 4 ft wide = 200 sq ft. At 4 inches: 2.47 cu yd.

Shed pad: 12 × 16 = 192 sq ft. At 4 inches: 2.37 cu yd.

Concrete is the one job on this site where square footage is only half the answer. Ready-mix is ordered by the cubic yard, so you need area and thickness. The conversion is area in square feet, times thickness in feet, divided by 27 — because a cubic yard is 3 × 3 × 3 ft.

Thickness is set by what the slab carries. Four inches is standard for a patio, walkway or shed base. Six inches is the norm for a driveway, anything a vehicle parks on, or a slab carrying point loads. Going from 4 to 6 inches is a 50% increase in concrete on the same footprint, which is the single biggest cost lever on a slab.

Worked example — a 20 × 20 ft patio at 4 inches. Area is 400 sq ft. Thickness is 4 ÷ 12 = 0.333 ft. Volume is 400 × 0.333 ÷ 27 = 4.94 cubic yards. At 6 inches the same patio needs 7.41 cubic yards.

Order more than the calculation says. Subgrade is never perfectly level, and the low spots quietly absorb concrete — most crews add 5 to 10% for a slab on prepared ground, more on rough ground. Running short mid-pour is the worst outcome available: you get a cold joint through the middle of the slab that will be visible and structurally weak forever.

Ready-mix is usually sold in quarter-yard increments with a short-load fee below about a yard, so a 4.94 yard order becomes 5.25 or 5.5 yards in practice. Confirm the increment with the plant when you book, and book the pour for the morning if you can — concrete placed in afternoon heat gives you far less working time.

Ready-mix concrete pricing varies by region and additives:

Standard 3,000 PSI concrete: $130-180 per cubic yard (2024 averages, delivered).

4,000 PSI (driveways, structural): $145-200 per cubic yard.

5,000 PSI (commercial, high-load): $170-225 per cubic yard.

Add cost per yard: fiber mesh reinforcement ($8-15/yard), color additive ($35-75/yard), accelerator for cold weather ($15-25/yard), retarder for hot weather ($8-15/yard).

Delivery fees: $50-200 per delivery, often with minimum yard requirements (3-5 yards typical minimum).

Total installed costs (materials + labor + finishing): $4-8 per sq ft for plain concrete, $8-15 for stamped or stained, $15-25 for decorative finishes.

The cubic yards above buy concrete. A slab also sits on something and is separated from the ground by something, and both are quantities you have to order — one by volume like the concrete, one by area.

The compacted subbase is the volume, and there is a shortcut worth remembering: at the same depth over the same area it is exactly the same figure as the concrete. The 20 × 30 ft patio at 4 in above is 7.4 cubic yards of concrete, and a 4 in gravel subbase under it is another 7.4 cubic yards of stone. Same area, same depth, same volume. What changes is the excavation, which now has to reach 8 in below finished level rather than 4, and that is 14.8 cubic yards of spoil to move.

The vapour retarder is the area, and it is the one place where the slab's square footage is not enough material. Sheeting is lapped at the seams and turned up at the edges, so a 600 sq ft slab needs meaningfully more than 600 sq ft of it. Buy to the roll size rather than to the slab, and check the lap the manufacturer specifies rather than assuming.

Where the retarder goes is a genuine decision rather than a detail. Directly under the slab it stops ground moisture rising into the concrete, which matters enormously if anything moisture-sensitive is going on top later — a wood floor, an adhesive, a coating. Under an exterior patio with nothing above it, it can be omitted. Adding one later is not possible without removing the slab.

None of these change the square footage. They change how many separate quantities that one measurement has to produce, which on a slab is three: concrete by volume, subbase by volume, and retarder by area with an allowance the area does not tell you.

Concrete is ordered before it is placed, against a depth you have not yet proved. Every other material on this site is bought against a surface you can measure; this one is bought against a hole, and holes are easy to get wrong in the expensive direction.

The arithmetic is unforgiving because the area is large and the depth is small. The 20 × 30 ft patio above is 600 sq ft at 4 in, or 7.41 cubic yards. Dig it an inch deeper than intended and the same slab needs 9.26 cubic yards — the extra inch is 50 cubic feet, 1.85 cubic yards, a full 25 percent more concrete. Half an inch is 12.5 percent. Two inches is half as much again.

It does not average out, either. A subgrade that is an inch low in one corner and an inch high in another does not cancel: the high spot leaves the slab thin exactly where you cannot see it, and the low spot swallows concrete. Both are defects, and only one of them shows up on the invoice.

This is why the depth is checked across the whole slab before the truck is booked, not on the day. Run a string line or a laser across the excavation at several points, measure down to the subgrade, and correct the high and low spots by adjusting the compacted base rather than by pouring more concrete. Base material is a fraction of the price and it is the layer designed to take up the tolerance.

Order to the measured depth and keep the difference in mind when the truck arrives. Running short mid-pour is worse than any over-order, because a cold joint through the middle of a slab is permanent — which is why the usual advice is to add a modest allowance on top of the calculated volume rather than to trust the excavation to the inch.