How the calculator works it out
Concrete is sold by volume. The calculator finds the volume of your pour, adds the waste allowance, then divides by how much one bag makes.
Slab: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet
Round column: 3.1416 × (diameter in inches ÷ 24)² × height (ft) = cubic feet
Cubic yards = cubic feet ÷ 27
Worked example: a 12×16 ft patio
12 × 16 × 4 ÷ 12 = 64 cubic feet. Add 10% for waste: 70.4 cubic feet, or 2.61 cubic yards. That is 118 bags of 80 lb mix. At that size, a short-load from a ready-mix supplier is usually the saner choice than mixing nearly 4,000 lb of bags by hand.
How much one bag makes
| Bag size | Makes | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cubic ft | 90 |
| 60 lb | 0.45 cubic ft | 60 |
| 80 lb | 0.60 cubic ft | 45 |
These are the yields printed on standard pre-mixed concrete bags such as Quikrete and Sakrete. Fast-setting post mix can yield slightly less, so check the bag.
Before you pour
- Measure the inside of your forms, not the outside of the boards.
- Thickness matters most. A slab that sags from 4 to 5 inches in the middle needs 25% more concrete there.
- For post holes, use the hole diameter, not the post. The calculator does not subtract the post, which gives you a small built-in margin.
Questions people ask
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10×10 ft slab needs 33.3 cubic feet (1.23 cubic yards). That is 56 bags of 80 lb mix, 75 bags of 60 lb or 112 bags of 40 lb, before any waste allowance. Add about 10% for spillage and an uneven base: 62 bags of 80 lb.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, walkways and shed floors. Driveways that carry cars are normally poured 5 to 6 inches thick, and heavier loads need an engineer. Always check your local building code.
Should I use bags or order ready-mix concrete?
Bags make sense for small jobs like fence posts, steps and slabs under about one cubic yard. Above that, mixing 50+ bags by hand is slow and the pour can set unevenly, so a ready-mix truck is usually easier and often not much more expensive.
How much extra concrete should I order?
Most contractors add 5 to 10 percent. Use the higher end if the ground is uneven, the forms might bow, or you are pouring by hand. Running short halfway through a pour leaves a weak cold joint.
How many cubic feet are in a cubic yard?
There are 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft). Ready-mix suppliers sell by the cubic yard; bagged mix is rated in cubic feet per bag.