Concrete Calculator
Volume = length × width × depth
How Concrete Volume Calculations Work
Concrete volume is measured in cubic feet (ft³) or cubic yards (yd³). For a rectangular slab, the formula is: volume = length × width × depth (all in feet). A standard 12 ft × 12 ft patio poured 4 inches deep needs depth converted to feet: 4 ÷ 12 = 0.333 ft. Volume = 12 × 12 × 0.333 = 48 cubic feet.
Contractors order ready-mix concrete by the cubic yard. Since 1 cubic yard = 27 cubic feet, divide cubic feet by 27: 48 ÷ 27 = 1.78 cubic yards. Always round up — a short pour is far worse than a small surplus. For the 12×12 patio, order 2 cubic yards to be safe.
Bagged concrete is sold in 60 lb and 80 lb sacks. A 60 lb bag yields about 0.45 cu ft; an 80 lb bag yields about 0.6 cu ft. For 48 cu ft, you need roughly 107 sixty-pound bags (48 ÷ 0.45) or 80 eighty-pound bags. Bagged mix suits small pads and post footings; ready-mix is economical above ~1 yd³.
Cylindrical columns use π × radius² × height. Stairs require summing the volume of each tread-and-riser step. Standard patio thickness is 4 inches for foot traffic; driveways need 4–6 inches with rebar. Add 5–10% extra for spillage, uneven subgrade, and pump hose waste on larger pours.
| Slab Size | Depth | Cubic Yards |
|---|---|---|
| 10 ft × 10 ft | 4 in | 1.23 yd³ |
| 12 ft × 12 ft | 4 in | 1.78 yd³ |
| 12 ft × 20 ft | 4 in | 2.96 yd³ |
| 20 ft × 20 ft | 6 in | 7.41 yd³ |
Before ordering, compact the subgrade and set forms square — a 12×12 slab that drifts to 12.5×11.5 changes volume noticeably. Enter your slab dimensions, column diameter, or stair details to get cubic feet, cubic yards, and bag counts instantly. Pair with the square footage calculator to verify your footprint area before pouring.
Examples
| Example | Result |
|---|---|
| 12×12 ft slab, 4 in deep | 48 cu ft, 1.78 yd³, ~107 bags (60 lb) |
| 10×10 ft slab, 4 in deep | 33.3 cu ft, 1.23 yd³ |
| 12×20 ft driveway section, 6 in | 120 cu ft, 4.44 yd³ |
| Round column, 12 in dia, 4 ft tall | 3.14 cu ft |
| 8×10 ft shed pad, 4 in deep | 26.7 cu ft, 0.99 yd³ |
| 15×15 ft patio, 4 in deep | 75 cu ft, 2.78 yd³ |
| 6×6 ft stepping pad, 3 in deep | 9 cu ft, 0.33 yd³ |
Frequently asked questions
Four inches is standard for patios and walkways. Driveways and areas with vehicle traffic require 4–6 inches with proper base compaction and reinforcement.
One cubic yard (27 cu ft) requires about 45 eighty-pound bags (27 ÷ 0.6). Sixty-pound bags need roughly 60 bags per cubic yard.
Yes — add at least 10% for waste on slabs and more for complex forms. Running short mid-pour can create cold joints and weak seams.