Concrete Calculator
Enter the size of your slab, footing or post holes to get the concrete in cubic yards and cubic feet, and how many bags to buy.
Concrete for common slab sizes
For a slab 4 inches thick, before waste:
| Slab | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 0.20 | 5.3 | 9 | 12 |
| 8 × 8 ft | 0.79 | 21.3 | 36 | 48 |
| 10 × 10 ft | 1.23 | 33.3 | 56 | 75 |
| 10 × 12 ft | 1.48 | 40.0 | 67 | 89 |
| 12 × 12 ft | 1.78 | 48.0 | 80 | 107 |
| 10 × 20 ft | 2.47 | 66.7 | 112 | 149 |
| 12 × 24 ft | 3.56 | 96.0 | 160 | 214 |
| 20 × 20 ft | 4.94 | 133.3 | 223 | 297 |
| 24 × 24 ft | 7.11 | 192.0 | 320 | 427 |
For a 6 inch slab, multiply by 1.5. Add 5–10% for waste before you buy.
How to calculate concrete
- Measure in feet. Convert the thickness to feet too: 4 in = 0.333 ft, 5 in = 0.417 ft, 6 in = 0.5 ft.
- Multiply length × width × thickness to get cubic feet. 10 × 10 × 0.333 = 33.3 cu ft.
- Divide by 27 to get cubic yards, the unit ready-mix is sold in: 33.3 ÷ 27 = 1.23 yd³.
- For bags, divide cubic feet by the bag yield: 0.6 for 80 lb, 0.45 for 60 lb, 0.3 for 40 lb. 33.3 ÷ 0.6 = 56 bags.
- Add 5–10% for spillage, uneven ground and forms that bow out.
For a round hole or column, the volume is 3.1416 × radius² × depth. Measure your forms and holes with a tape measure, and for reinforcing bar sizes see the rebar size chart.
Bags or ready-mix?
| Job size | Best option |
|---|---|
| Under ½ cubic yard (fence posts, small pads, steps) | Bags, mixed in a wheelbarrow or small mixer |
| ½ to 1 cubic yard | Bags with a rented mixer, or a trailer of ready-mix from a yard |
| Over 1 cubic yard (most patios and driveways) | Ready-mix truck. One yard is 45 bags of 80 lb, over 3,600 lb to mix by hand |
Many ready-mix suppliers have a minimum order or charge a short-load fee under a few yards: ask when you order.
Typical thickness
| Project | Thickness |
|---|---|
| Walkway, patio, shed floor | 4 in |
| Driveway or garage floor (cars) | 5–6 in |
| Hot tub or heavy equipment pad | 6 in or more |
| Fence post holes | 3× the post width across; ⅓ of the post length deep or below the frost line |
These are common starting points, not a structural design: footings, foundations and anything that carries a building should follow local code.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. That takes about 56 bags of 80 lb concrete (0.6 cu ft each) or 75 bags of 60 lb, before waste. Add about 10% for spillage and uneven ground: around 62 bags of 80 lb. At that size, ordering ready-mix is usually easier.
How many cubic yards of concrete do I need?
Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27. For a 12 × 12 ft slab 4 inches thick: 12 × 12 × 0.333 = 48 cu ft, and 48 ÷ 27 = 1.78 cubic yards.
How much concrete does one 80 lb bag make?
About 0.6 cubic feet. It takes 45 bags of 80 lb concrete to make one cubic yard, 60 bags of 60 lb or 90 bags of 40 lb. Yields vary a little by brand, so check the bag.
How thick should a concrete slab be?
4 inches is standard for patios, walkways and shed floors. Driveways and garage floors for cars are usually 5 to 6 inches, and slabs for heavy trucks or equipment thicker still. Local codes and soil conditions may require more.
How many bags of concrete for a fence post?
For a 4x4 post in a 10 inch wide hole 2 feet deep, about 0.92 cubic feet: one and a half 80 lb bags (buy 2) or just over two 60 lb bags (buy 3). A common rule is a hole three times the post width and one third of the post length deep (or below the frost line).
Spreading something on top? See the gravel calculator for a base layer, or the mulch calculator for garden beds.