Concrete Calculator
Enter the size of your slab, footing or column. You get the volume in cubic yards for a ready-mix order and the number of premix bags if you are mixing it yourself.
- 80 lb bags
- 62
- 60 lb bags
- 82
- 50 lb bags
- 98
- 40 lb bags
- 123
- Ready-mix cost
- —
How the math works
Slab: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet
Column: π × (diameter ÷ 24)² × height (ft) = cubic feet
Cubic yards = cubic feet ÷ 27
Example: a 10 × 10 ft patio, 4 inches thick, is 10 × 10 × 4 ÷ 12 = 33.33 cu ft, which is 1.23 cubic yards before waste. With 10 % extra you would order 1.36 yards. Most plants deliver in quarter- or half-yard steps, so round up.
Bags or ready-mix?
Bag counts use the yields printed on premix bags: an 80 lb bag makes 0.60 cu ft, 60 lb makes 0.45, 50 lb makes 0.375 and 40 lb makes 0.30. One cubic yard therefore takes 45 bags of 80 lb. That is 3,600 lb of mixing by hand. Above roughly one yard, a ready-mix truck or a rental mixer is usually cheaper and gives a stronger, more even slab. Check the minimum order and short-load fee with your local plant.
Common slabs at 4 inches
| Slab | Cu yd | +10 % | 80 lb bags |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.36 | 62 |
| 12 × 12 ft | 1.78 | 1.96 | 88 |
| 10 × 20 ft | 2.47 | 2.72 | 123 |
| 20 × 20 ft (2-car pad) | 4.94 | 5.43 | 245 |
| 24 × 24 ft (garage) | 7.11 | 7.82 | 352 |
How thick should it be?
Typical thicknesses are 4 in for patios, walkways and shed floors, 5–6 in for driveways and garage floors, and 6 in or more where heavy trucks or RVs will park. Footing size depends on your soil and the load, so local code sets it. Use the slab mode for a continuous footing, with length as the total run and thickness as its depth.
Add 5–10 % for spillage, uneven subgrade and form bowing. Add more if the ground has not been graded flat. Running short in the middle of a pour leaves a cold joint.