Concrete slab calculator

A 10 × 10 ft slab 4 in thick needs 1.23 cubic yards of concrete, or 1.36 yards with 10% extra: 62 bags of 80 lb mix or 82 bags of 60 lb. Enter your slab's size for your own numbers.

Length

Width

Thickness

Extra for waste

1.36 cubic yards

62 80 lb bags

82 60 lb bags

Volume	33.33 cu ft (1.23 cu yd, 0.944 m³)
With 10% extra	36.67 cu ft (1.36 cu yd)
Ready-mix to order	1.5 cu yd (rounded up to the next ¼ yard)
40 lb bags	123 (0.3 cu ft each)
50 lb bags	98 (0.375 cu ft each)
60 lb bags	82 (0.45 cu ft each)
80 lb bags	62 (0.6 cu ft each)
90 lb bags	55 (0.675 cu ft each)
Weight of the concrete	about 5,317 lb (2.66 tons)

How it was worked out

10 ft × 10 ft × 4 in = 33.33 cu ft

33.33 cu ft ÷ 27 = 1.23 cu yd

Measuring the slab

Measure the inside of the forms, not the outside, and take the thickness from the top of the form to the prepared base in a few places. Ground is rarely flat: if one corner is half an inch lower than you thought, that corner takes more concrete. The National Ready Mixed Concrete Association gives the example of a 4 in slab that ends up 1/8 in thicker than planned, which is a 3 percent shortfall, and advises ordering at least 4 to 10 percent more than the measured volume. The calculator adds 10% unless you choose otherwise.

For an L-shaped patio, split it into two rectangles and add the results. For a round slab, use the column calculator with the slab's diameter and its thickness as the height.

Worked example: a 12 × 20 ft driveway pad, 5 in thick

12 ft × 20 ft × 5 in = 100 cu ft

100 cu ft ÷ 27 = 3.70 cu yd

With 10% extra: 4.07 cu yd, or 184 bags of 80 lb mix

At that size, ready-mix is the sensible choice: the calculator rounds the order up to 4.25 cubic yards.

Common slab sizes at 4 in thick

Concrete for a 4 in slab
Slab	Cu yd (exact)	Cu yd with 10%	80 lb bags	60 lb bags
4 × 4 ft	0.20	0.22	10	14
4 × 8 ft	0.40	0.43	20	27
6 × 6 ft	0.44	0.49	22	30
8 × 8 ft	0.79	0.87	40	53
8 × 10 ft	0.99	1.09	49	66
10 × 10 ft	1.23	1.36	62	82
10 × 12 ft	1.48	1.63	74	98
10 × 20 ft	2.47	2.72	123	163
12 × 12 ft	1.78	1.96	88	118
12 × 20 ft	2.96	3.26	147	196
16 × 16 ft	3.16	3.48	157	209
20 × 20 ft	4.94	5.43	245	326
20 × 24 ft	5.93	6.52	294	392
24 × 24 ft	7.11	7.82	352	470
30 × 30 ft	11.11	12.22	550	734

Concrete per square foot

Each inch of thickness adds 1/12 cu ft for every square foot of slab. One cubic yard covers 324 sq ft at 1 in, so divide 324 by the thickness in inches to see how far a yard goes.

Coverage by slab thickness, no extra included
Thickness	Cu ft per sq ft	Sq ft per cu yd	80 lb bags per 100 sq ft
3.5 in	0.292	92.6	49
4 in	0.333	81	56
5 in	0.417	64.8	70
6 in	0.5	54	84
8 in	0.667	40.5	112

How thick to pour

The International Residential Code asks for concrete floor slabs at least 3½ in thick. Patios, walks and shed bases are commonly 4 in. Driveways and slabs that carry heavy vehicles or equipment are usually poured thicker, and many towns set their own minimum for driveways, so check with your building department before you order. A thicker slab also needs a well compacted base under it; the crushed stone calculator works out the gravel for that layer.

Sources

CIP 8: Discrepancies in Yield (Concrete in Practice series), National Ready Mixed Concrete Association, 2016 edition. Order at least 4 to 10 percent more than the measured volume; 1/8 in extra on a 4 in slab is a 3 percent shortfall.
International Residential Code, sections R403.1.1 and R506.1, International Code Council, 2021 edition. Spread footings at least 6 in thick with projections of 2 in or more; Table R403.1 widths start at 12 in; concrete floor slabs at least 3.5 in thick.
QUIKRETE Concrete Mix (No. 1101) data sheet, The QUIKRETE Companies, revised October 2022. Bag yields: 40 lb 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60, 90 lb 0.675; about 140 lb per cubic foot.