The calculator adds 10% extra by default to cover uneven ground, spillage and form flex. Running short halfway through a pour is far worse than having a bag left over.
How to use the concrete calculator
- Choose a slab (patio, footing, driveway, shed base) or post holes.
- Enter the length and width in feet and the thickness in inches. For post holes, enter the hole diameter, depth and how many holes.
- Keep the 10% waste allowance unless your forms and ground are perfectly level.
- Optionally open "Prices" and enter your local bag and ready-mix prices for an accurate cost.
The formula
Slab: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet. Divide by 27 to get cubic yards.
Post hole: π × (diameter ÷ 24)² × depth (in) ÷ 12 × number of holes = cubic feet.
Bags: one 80 lb bag makes about 0.60 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft.
Concrete needed for common slab sizes (4 in thick, 10% extra)
| Slab size | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft | 1.36 | 62 | 82 |
| 12 × 12 ft | 1.96 | 88 | 118 |
| 10 × 20 ft | 2.72 | 123 | 163 |
| 16 × 16 ft | 3.48 | 157 | 209 |
| 20 × 20 ft | 5.43 | 245 | 326 |
| 24 × 24 ft | 7.82 | 352 | 470 |
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45 bags. One cubic yard is 27 cubic feet, and each 80 lb bag makes roughly 0.60 cubic feet.
How thick should a concrete slab be?
Patios, walkways and shed bases are usually 4 inches thick. Driveways and slabs that carry vehicles are typically 5 to 6 inches. Check local building codes for structural work.
Should I buy bags or order a ready-mix truck?
Bags are practical up to about 1 cubic yard (around 45 bags of 80 lb). Beyond that, a truck usually costs less and saves hours of mixing, although small loads may have a minimum charge.
How much concrete do I need for a fence post?
A typical 4×4 post in a 10 inch wide, 30 inch deep hole needs about 2 to 3 bags of 80 lb concrete. Choose "Post holes" above to calculate your exact size and number of posts.
Results are estimates for planning and budgeting. Always confirm measurements and product coverage before you buy, and follow local building codes.