How to use it
Pick the shape, type the size in any unit, and set how many you are pouring, for example 8 post holes. Measuring in feet and inches? Type the inches in the small box next to feet.
The formula
Volume (cu ft) = length × width × thickness (all in feet) Round holes: 3.1416 × (diameter ÷ 2)² × depth Cubic yards = cubic feet ÷ 27
Example: a 10 × 10 ft patio, 4 inches thick, is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. With 10% extra that is about 1.36 cubic yards, or 62 bags of 80 lb.
Quick answers for common sizes
| Slab, 4 in thick | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 0.2 | 10 | 14 | 20 |
| 8 × 8 ft | 0.79 | 40 | 53 | 79 |
| 10 × 10 ft | 1.23 | 62 | 82 | 123 |
| 12 × 12 ft | 1.78 | 88 | 118 | 176 |
| 10 × 20 ft | 2.47 | 123 | 163 | 245 |
| 16 × 16 ft | 3.16 | 157 | 209 | 313 |
| 20 × 20 ft | 4.94 | 245 | 326 | 489 |
| 24 × 24 ft | 7.11 | 352 | 470 | 704 |
| Post hole | Concrete | 80 lb bags | 50 lb bags |
|---|---|---|---|
| 6 in wide, 2 ft deep | 0.39 cu ft | 1 | 2 |
| 8 in wide, 2 ft deep | 0.7 cu ft | 2 | 3 |
| 8 in wide, 3 ft deep | 1.05 cu ft | 2 | 4 |
| 10 in wide, 3 ft deep | 1.64 cu ft | 3 | 5 |
| 12 in wide, 3 ft deep | 2.36 cu ft | 5 | 7 |
| 12 in wide, 4 ft deep | 3.14 cu ft | 6 | 10 |
How many bags make a cubic yard
| Bag size | Concrete per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Tips before you pour
- 4 inches is the usual thickness for patios, sidewalks and shed floors; driveways are usually 5 to 6 inches.
- Add 5 to 10 percent for spills, bulging forms and uneven ground. The calculator adds 10% unless you change it.
- Bags suit small jobs like post holes and pads. Past about a cubic yard (45 bags of 80 lb), ready-mix delivery is usually easier; suppliers sell it in quarter-yard steps.
Sources and assumptions
Bag yields are the figures printed on standard premixed concrete bags: 0.30, 0.375, 0.45 and 0.60 cubic feet for 40, 50, 60 and 80 lb. Weight uses 150 lb per cubic foot, the usual figure for normal-weight concrete. Ready-mix is rounded up to the next quarter yard.
Questions
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb. A cubic yard is 27 cubic feet, and an 80 lb bag makes about 0.6 cubic feet.
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, 62 bags of 80 lb or 82 bags of 60 lb, including 10 percent extra. That is 1.23 cubic yards.
How many bags of concrete per fence post?
For a 10-inch hole 3 feet deep, about 3 bags of 80 lb. A 12-inch hole 3 feet deep takes about 5.
How much does a cubic yard of concrete weigh?
About 4,000 lb, or 2 tons. Normal concrete weighs roughly 150 lb per cubic foot, and a cubic yard holds 27 cubic feet.
How thick should a concrete slab be?
4 inches for patios, walkways and shed floors. Driveways usually need 5 to 6 inches, more if heavy trucks use them.