Calculate concrete volume in cubic feet (and cubic yards) for any rectangular or custom-shaped pour.
Cubic Feet vs Cubic Yards — When to Use Which
Cubic feet and cubic yards measure the same thing — volume — but at different scales. Choose the right unit for your project:
- Use cubic feet when: Buying bagged concrete (bag yield is listed in cu ft), small projects under 1 cu yd, sonotube and footing math, mixing concrete by hand in a wheelbarrow.
- Use cubic yards when: Ordering ready-mix delivery (plants quote by the yard), large pours over 1 cubic yard, comparing concrete cost estimates from contractors.
The conversion is simple: 27 cubic feet = 1 cubic yard. A 10×10 patio at 4 inches thick is 33.3 cubic feet OR 1.23 cubic yards — same volume, different unit.
Cubic Feet Quick Reference
| Slab Size (4-inch depth) | Cubic Feet | Cubic Yards | 80lb Bags (w/ buffer) |
|---|---|---|---|
| 3×3 ft | 3.0 | 0.11 | 6 |
| 5×5 ft | 8.33 | 0.31 | 16 |
| 10×10 ft | 33.3 | 1.23 | 62 |
| 10×15 ft | 50.0 | 1.85 | 92 |
| 15×15 ft | 75.0 | 2.78 | 138 |
| 20×20 ft | 133.3 | 4.94 | 244 |
Bag Yield in Cubic Feet
Concrete bags yield different volumes when mixed with water. Knowing the cubic-feet yield per bag lets you order exactly the right number without doing yardage conversion:
| Bag Size | Yield (cu ft) | Bags per cu yd (raw) | Bags per cu yd (with 10%) |
|---|---|---|---|
| 40 lb | 0.30 | 90 | 99 |
| 60 lb | 0.45 | 60 | 66 |
| 80 lb | 0.60 | 45 | 50 |
Cubic Feet for Non-Slab Pours
Not every concrete pour is a rectangular slab. Common project shapes and their cubic-feet math:
- Sonotube column: π × radius² × depth, all in feet. A 10-inch sonotube at 4 ft deep = 3.14 × 0.417² × 4 = 2.18 cu ft.
- Strip footing: Same as a slab — length × width × depth. A 20-ft footing × 18 in wide × 12 in deep = 20 × 1.5 × 1.0 = 30 cu ft.
- Stair step: Calculate each step as a rectangle, then sum. A 3-step stair with 36-inch wide × 11-inch tread × 7-inch riser averages 8-10 cu ft total.
- Curb or edge beam: length × width × depth. A 20-ft curb × 6 in wide × 6 in tall = 20 × 0.5 × 0.5 = 5 cu ft.
For complex pour shapes (L-shape patios, irregular driveways), break the area into rectangles and circles, calculate each section separately, then add the cubic feet together. Apply the 10% buffer to the total — not to each section individually.
Quick Verification Method
After calculating cubic feet, do a sanity check before ordering. Multiply your cubic feet by 0.037 — the result should match your cubic yards within 0.05. If the numbers do not match, recheck your inch-to-foot conversion on the thickness measurement. Most calculation errors come from forgetting to divide depth in inches by 12 to get feet.