How the calculator works
Concrete is sold by volume. Bags are rated in cubic feet of mixed concrete; ready-mix trucks sell by the cubic yard, which is 27 cubic feet (3 × 3 × 3). The calculator finds the volume of each piece in cubic feet, adds them up, adds your waste allowance, then converts.
| Shape | Cubic feet = |
|---|---|
| Slab, patio, walkway | length (ft) × width (ft) × thickness (in) ÷ 12 |
| Footing, trench, curb | length (ft) × width (in) ÷ 12 × depth (in) ÷ 12 |
| Round column or hole | 3.1416 × (diameter ÷ 2)² × depth, all in inches, ÷ 1,728 |
| Solid steps on grade | width × run × rise × n(n+1) ÷ 2, in feet, for n steps |
| Cubic yards | cubic feet ÷ 27 |
| Bags | cubic feet ÷ yield per bag, rounded up |
Steps are treated as a stack of solid blocks sitting on the ground: the first step is one rise tall, the second is two rises tall, and so on, each one run deep. If your steps sit on a slab or have a deeper top landing, add that landing as a separate slab piece.
Worked example: a 10 × 10 patio
- Volume: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet = 1.23 cubic yards
- With 10% extra: 36.7 cubic feet = 1.36 cubic yards
- 80 lb bags at about 0.60 cubic feet each: 36.7 ÷ 0.60 = 61.1, rounded up to 62 bags (4,960 lb)
- 60 lb bags at about 0.45 cubic feet: 82 bags
Add three steps up to the patio, 4 feet wide with a 7-inch rise and 11-inch run: 4 × (11 ÷ 12) × (7 ÷ 12) × 6 = 12.8 cubic feet more. Use the “Add another piece” button to total both.
Bag yields
QUIKRETE's data sheet for its standard Concrete Mix lists these approximate yields. Other brands, high-strength mixes and fast-setting products can differ, so choose “Other” and type in the yield printed on your bag when it does not match.
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | ≈0.30 ft³ | 90 |
| 50 lb | ≈0.375 ft³ | 72 |
| 60 lb | ≈0.45 ft³ | 60 |
| 80 lb | ≈0.60 ft³ | 45 |
| 90 lb | ≈0.675 ft³ | 40 |
Bags or ready-mix?
Bags make sense for post holes and small pads. As the job grows, so does the labor: a cubic yard of 80-lb bags is 45 bags and 3,600 pounds to lift and mix. Ready-mix arrives in one load, but suppliers may set minimum orders and add small-load and delivery fees. Enter your real quotes and the calculator compares the two.
What to do next
For fence posts, the fence calculator counts posts and concrete per hole. For a deck, the deck calculator counts footings you can then enter here as round columns. In New York City, check whether your project needs a DOB permit.