Concrete Calculator

By Stephen Morrison, Editor Last reviewed October 30, 2026

Enter your slab or hole size and get cubic yards, bag counts for every common bag size and a ready-mix order. The math is shown below.

Quick answer: a 10 × 10 ft slab 4 inches thick needs 33.33 cubic feet of concrete. Add 10% extra and that's1.36 cubic yards: 62 bags of 80 lb mix or 82 bags of 60 lb mix.

How the calculator works

Concrete is sold by volume, so everything starts with cubic feet. Thickness is in inches, so we divide it by 12 to get feet.

Slab: volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
Column: volume (ft³) = π × (diameter (in) ÷ 24)² × depth (ft)
Total: volume × quantity × (1 + waste% ÷ 100)
Yards: total ft³ ÷ 27
Bags: total ft³ ÷ bag yield, rounded up

Ready-mix is rounded up to the next quarter yard, the smallest step most plants sell.

Bags per cubic yard

Bag yields are from the QUIKRETE Concrete Mix data sheet. Other brands of standard concrete mix are similar, but check the yield printed on your bag.

Bag size	Yield per bag	Bags per cubic yard
80 lb	0.6 ft³	45
60 lb	0.45 ft³	60
50 lb	0.375 ft³	72
40 lb	0.3 ft³	90

Concrete for common slab sizes

All at 4 inches thick with 10% extra.

Slab	Area	Cubic yards	80 lb bags	60 lb bags
10 × 10 ft	100 ft²	1.36	62	82
12 × 12 ft	144 ft²	1.96	88	118
10 × 20 ft	200 ft²	2.72	123	163
16 × 16 ft	256 ft²	3.48	157	209
20 × 20 ft	400 ft²	5.43	245	326
24 × 24 ft	576 ft²	7.82	352	470

How thick should a slab be?

Four inches is the usual thickness for patios, walkways and shed floors. Driveways and slabs that carry vehicles are usually poured thicker. Your local building code and the load the slab will carry decide the real number, so check before you pour. Every extra inch adds 25% to a 4-inch slab's concrete.

Measuring tips

Measure the inside of your forms, not the outside.
For an uneven base, measure depth in a few spots and use the average.
For an L-shaped patio, work out each rectangle separately and add them. The square footage calculator handles that.
For post holes, the post takes up space. A 4 × 4 post (3.5 × 3.5 in actual) displaces about 0.085 ft³ per foot of depth.

Common questions

How many 80 lb bags of concrete make a cubic yard?

45. A cubic yard is 27 cubic feet and an 80 lb bag yields about 0.60 cubic feet, so 27 ÷ 0.60 = 45. For 60 lb bags it's 60, and for 40 lb bags it's 90.

How many bags do I need for a 10 × 10 slab?

At 4 inches thick with 10% extra, a 10 × 10 ft slab needs about 1.36 cubic yards: 62 bags of 80 lb mix or 82 bags of 60 lb mix.

How many bags for a fence post hole?

A 10 inch wide, 3 ft deep hole holds about 1.64 cubic feet, which is 3 bags of 80 lb mix or 5 bags of 50 lb mix before you subtract the post. Use the "Round column or hole" option above and set the number of holes.

Should I order ready-mix or use bags?

It comes down to labor. One cubic yard is 45 bags of 80 lb mix, about 3,600 lb to lift and mix by hand. Past a yard or so, many DIYers order ready-mix instead. Small orders often carry a short-load fee, so call your local plant for a price.

Why add 5–10% extra?

Ground is never perfectly flat, forms bow slightly and some concrete always gets spilled or left in the mixer. Running short halfway through a pour leaves a weak cold joint, so a small surplus is cheap insurance.

Is cement the same as concrete?

No. Cement is the powder that binds concrete together. Concrete is cement mixed with sand, gravel and water. Bagged "concrete mix" already contains everything except the water.