Concrete Slab Calculator: Yards, Bags and Cost

How to calculate concrete for a slab

Multiply length by width by thickness, with everything in feet, to get cubic feet. Divide by 27 to get cubic yards, the unit ready-mix is sold in.

Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27

For a 10 × 10 ft slab at 4 inches: 10 × 10 × (4 ÷ 12) = 33.3 cu ft. Divided by 27, that is 1.23 cubic yards. For bags, divide the cubic feet by the bag yield: an 80 lb bag makes about 0.6 cu ft, so you need 56 bags.

Then add some extra; the calculator defaults to 10%. Subgrade is never perfectly flat, and a slab that's half an inch thicker than planned eats concrete fast.

Concrete for common slab sizes

Before waste. Ready-mix cost uses the national range of $160–$195 per cubic yard, not including delivery fees.

Slab	Area	4 in thick	6 in thick	80 lb bags (4 in)	Ready-mix (4 in)
4 × 4 ft	16 sq ft	0.20 cu yd	0.30 cu yd	9	$32–$39
10 × 10 ft	100 sq ft	1.23 cu yd	1.85 cu yd	56	$200–$240
12 × 12 ft	144 sq ft	1.78 cu yd	2.67 cu yd	80	$285–$345
10 × 20 ft	200 sq ft	2.47 cu yd	3.70 cu yd	112	$395–$480
20 × 20 ft	400 sq ft	4.94 cu yd	7.41 cu yd	223	$790–$965
24 × 24 ft	576 sq ft	7.11 cu yd	10.67 cu yd	320	$1,140–$1,390
30 × 40 ft	1200 sq ft	14.81 cu yd	22.22 cu yd	667	$2,370–$2,890

Suppliers charge extra for small loads. See concrete price per yard for short-load fees.

How thick should it be?

Concrete Network gives 4 inches as the minimum for a residential driveway carrying passenger cars and SUVs, and 5 to 6 inches for trucks or RVs. It notes that going from 4 to 5 inches raises load capacity by nearly 50%.^[6]

Thickness is a structural decision. For a garage, a shed that carries heavy equipment, or anything on poor soil, follow your local code or a contractor's advice.

Frequently asked questions

How much concrete do I need for a 10×10 slab?

At 4 inches thick, a 10 × 10 ft slab needs 1.23 cubic yards (33.3 cu ft), before waste. That is 56 bags of 80 lb concrete. Order about 10% extra.

How many cubic yards is a 20×20 slab?

A 20 × 20 ft slab needs 4.94 cubic yards at 4 inches and 7.41 at 6 inches, before waste.

How thick should a concrete slab be?

Concrete Network puts the minimum for a residential driveway at 4 inches for passenger cars, and 5 to 6 inches for trucks or RVs. Your local code and soil decide the real answer for anything structural.

Should I use bags or ready-mix?

Compare one cubic yard: 45 bags of 80 lb cost about $265–$290, against $160–$195 for a yard of ready-mix before delivery fees. Bags suit small jobs; for bigger pours, ready-mix is cheaper and far less mixing. See the bags of concrete calculator.

How much extra concrete should I order?

The calculator adds 10% by default, for spillage, uneven subgrade and forms that bow. You can set 0%, 5% or 15% instead. Running short mid-pour is worse than a little left over.

Sources

Concrete Network, Concrete prices, updated 2026-07-14. Checked September 23, 2026.

Quikrete, Concrete Mix No. 1101 data sheet, updated 2022-10. Checked September 23, 2026.

The Home Depot, Quikrete 80 lb Concrete Mix. Figure as shown in the page's search listing; checked September 23, 2026.

The Home Depot, Sakrete 80 lb Gray Concrete Mix. Figure as shown in the page's search listing; checked September 23, 2026.

This Old House, Concrete slab cost, updated 2026-03-17. Checked September 23, 2026.

Concrete Network, How thick should a concrete driveway be?. Figure as shown in the page's search listing; checked September 23, 2026.