How to use this concrete calculator
- Pick the shape: a flat slab or patio, a footing or wall, or a round post hole or column.
- Measure length and width in feet and the thickness in inches. You can switch any field to inches, yards, meters or centimeters.
- If you are pouring several identical pieces, such as 8 fence post holes, set How many.
- Keep the waste allowance at 5 to 10% unless your forms are perfectly square and level.
The formula
For a slab or footing, volume in cubic feet is length (ft) × width (ft) × thickness (in) ÷ 12. For a round column it is π × (diameter ÷ 2)² × height, with the diameter converted to feet first.
Divide cubic feet by 27 to get cubic yards, the unit ready-mix plants sell by. To get bags, divide cubic feet by the bag's yield and round up: about 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb, per Quikrete's and Sakrete's published data.
Worked example: a 10 × 10 ft patio, 4 inches thick
10 × 10 × 4 ÷ 12 = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards. Add 10% and you need 36.7 cubic feet. That is 36.7 ÷ 0.60 = 61.1, so buy 62 bags of 80 lb concrete.
Bags needed for common slab sizes
All rows assume a 4 inch thick slab with 10% extra.
| Slab size | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 0.22 | 10 | 14 | 20 |
| 6 × 6 ft | 0.49 | 22 | 30 | 44 |
| 8 × 8 ft | 0.87 | 40 | 53 | 79 |
| 10 × 10 ft | 1.36 | 62 | 82 | 123 |
| 10 × 12 ft | 1.63 | 74 | 98 | 147 |
| 12 × 12 ft | 1.96 | 88 | 118 | 176 |
| 10 × 20 ft | 2.72 | 123 | 163 | 245 |
| 12 × 24 ft | 3.91 | 176 | 235 | 352 |
| 20 × 20 ft | 5.43 | 245 | 326 | 489 |
| 24 × 24 ft | 7.82 | 352 | 470 | 704 |
Tips before you pour
- Measure the forms, not the plan. Measure inside the forms after they are set. An extra half inch of depth across a large slab adds up fast.
- Check local rules. Footing depth, slab thickness and reinforcement for structural work are set by your local building code.
- Weight matters. Concrete weighs roughly 140 to 150 lb per cubic foot. Sixty 80 lb bags is 4,800 lb, which is more than most pickup trucks should carry in one trip.