Cubic Feet Calculator
Calculate cubic feet for small projects: post holes, planters, sonotubes, small slabs, and any fraction-of-a-yard volume where cubic yards would be overkill.
Cubic Feet Calculator
Enter project dimensions below — results update instantly. Switch units freely.
Estimates assume typical industry density and waste factors. Always verify with your supplier and local building code before purchasing material.
When Cubic Feet Beats Cubic Yards
Cubic yards are the industry standard for bulk ordering, but for small projects cubic feet is more intuitive and more useful:
- Bagged concrete coverage — sold by cubic feet per bag yield
- Post holes, sonotubes — all under 1 yd³, typically 0.3-3 ft³
- Planter volumes — raised bed fills
- Small concrete pours — walkway sections, pad extensions
- Bag quantities — mulch, soil, and concrete mix all labeled in ft³
Bag yields to memorize:
- 40-lb concrete bag = 0.30 ft³
- 60-lb concrete bag = 0.45 ft³
- 80-lb concrete bag = 0.60 ft³
- 2-ft³ mulch bag = standard bag size
- 40-lb topsoil bag = ~0.5 ft³
Volume Formulas for Cubic Feet
Cylindrical: V = π × r² × h
Triangular prism: V = (b × h ÷ 2) × length
All in feet (convert inches by dividing by 12)
Cubic feet conversions:
- 1 ft³ = 1,728 in³ = 7.48 gallons = 28.32 liters
- 1 ft³ = 0.03704 yd³ = 0.02832 m³
- 1 yd³ = 27 ft³
- 1 m³ = 35.31 ft³
| Project | Dimensions | Volume | Bags 80-lb |
|---|---|---|---|
| Fence post hole (8 in × 2 ft) | 0.35 ft² × 2 ft | 0.70 ft³ | 2 |
| Mailbox post (10 in × 3 ft) | 0.55 ft² × 3 ft | 1.64 ft³ | 3 |
| Deck pier (12 in × 4 ft) | 0.79 ft² × 4 ft | 3.14 ft³ | 6 |
| Sonotube (12 in × 5 ft) | 0.79 ft² × 5 ft | 3.93 ft³ | 7 |
| Sidewalk panel (3 × 8 × 4 in) | 24 ft² × 0.33 ft | 8.0 ft³ | 14 |
| Stoop (4 × 5 × 4 in) | 20 ft² × 0.33 ft | 6.7 ft³ | 12 |
| Shower curb (6 × 6 in × 4 ft) | 0.25 ft² × 4 ft | 1.0 ft³ | 2 |
| Trench (2 × 6 in × 20 ft) | 1 ft² × 20 ft | 20 ft³ | 34 |
| Small footing (16 × 16 × 8 in) | 1.78 ft² × 0.67 ft | 1.19 ft³ | 2 |
80-lb bag of concrete yields 0.60 ft³. Round up bag counts. For volumes above 27 ft³ (1 yd³), consider ready-mix instead of bagged.
| Bag Type | Yield (ft³) | Bags per ft³ | Bags per yd³ (27 ft³) |
|---|---|---|---|
| 40-lb concrete | 0.30 | 3.33 | 90 |
| 60-lb concrete | 0.45 | 2.22 | 60 |
| 80-lb concrete | 0.60 | 1.67 | 45 |
| 50-lb mortar | 0.50 | 2.00 | 54 |
| 2-ft³ mulch bag | 2.00 | 0.50 | 13.5 |
| 3-ft³ mulch bag | 3.00 | 0.33 | 9 |
| 40-lb topsoil | 0.50 | 2.00 | 54 |
| 1-ft³ potting soil | 1.00 | 1.00 | 27 |
Always round up bag count. Return unused bags (most retailers accept sealed returns) rather than running short mid-project.
Real-World Example Calculations
Sonotube 12 in × 4 ft
Single deck support pier volume.
- Diameter
- 12 in (1 ft)
- Height
- 4 ft
Takeaway: 6 × 80-lb bags concrete. Order 7 to have margin.
Raised Bed 4 × 8 × 12 in
Raised garden bed fill with premium soil mix.
- Length × Width
- 8 × 4 ft
- Depth
- 12 in (1 ft)
Takeaway: 1.19 yd³ (right at the break between bags and bulk). Choose by supplier minimums; bagged premium mix averages $6/ft³ = $192; bulk $55/yd³ = $65.
Retaining Wall Drain 40 × 1 × 1 ft
Gravel drain zone directly behind retaining wall.
- Length × Width
- 40 × 1 ft
- Depth
- 12 in
Takeaway: 1.48 yd³ — order 1.5 yd³ of #57 stone bulk delivered.
Frequently Asked Questions
How do I calculate cubic feet?
Multiply length × width × height (all in feet). For inches: divide by 12 first to get feet. Example: a 24 × 36 × 4 in area = 2 × 3 × 0.333 = 2.0 ft³.
How many cubic feet in a cubic yard?
Exactly 27 cubic feet. Convert yd³ to ft³ by multiplying by 27; convert ft³ to yd³ by dividing by 27.
How many cubic feet in a bag of concrete?
80-lb bag = 0.60 ft³. 60-lb bag = 0.45 ft³. 40-lb bag = 0.30 ft³. These are the most common bag sizes at US home centers.
How many cubic feet in a gallon?
One cubic foot holds ~7.48 US gallons. Reverse: 1 gallon = 0.134 ft³. Useful for pool and irrigation calculations.
How do I calculate cubic feet of a cylinder?
V = π × r² × h where r is radius in feet, h is height in feet. Example: 10-in diameter (radius 5 in = 0.417 ft) × 4 ft = π × 0.174 × 4 = 2.18 ft³.
Should I order by cubic feet or cubic yards?
Cubic feet for small projects (<1 yd³ = <27 ft³) that use bagged materials. Cubic yards for bulk deliveries (ready-mix, gravel, mulch). The break-even is roughly 1 yd³ = 27 ft³ — below that, bags are more convenient and often cheaper per unit.
How do I convert cubic feet to cubic meters?
Multiply by 0.02832. Example: 100 ft³ = 2.83 m³. Reverse: m³ × 35.31 = ft³.