Round column formula
solid column: V = π × (D ÷ 2)² × H × count
hollow tube: V = π × ((D_out ÷ 2)² − (D_in ÷ 2)²) × H × count
D in inches → feet: D ÷ 12
Worked example: one 12 in tube, 4 ft tall
- Radius: 12 ÷ 2 = 6 in = 0.5 ft.
- Area: π × 0.5² = 0.785 ft².
- Volume: 0.785 × 4 = 3.14 ft³ = 0.116 yd³.
- 80 lb bags: 3.14 ÷ 0.60 = 5.2, so 6 bags.
Concrete per foot of tube
Net volume per foot of height, no waste. 80 lb bags at 0.60 ft³ .
| Diameter | ft³ per ft | 80 lb bags per ft | Feet per yd³ |
|---|---|---|---|
| 6 in | 0.196 | 0.33 | 137.5 |
| 8 in | 0.349 | 0.58 | 77.3 |
| 10 in | 0.545 | 0.91 | 49.5 |
| 12 in | 0.785 | 1.31 | 34.4 |
| 14 in | 1.069 | 1.78 | 25.3 |
| 16 in | 1.396 | 2.33 | 19.3 |
| 18 in | 1.767 | 2.95 | 15.3 |
| 24 in | 3.142 | 5.24 | 8.6 |
Post hole concrete calculator
For a fence or deck post, treat the hole as a column, then subtract the post. Square post volume = width × width × buried depth. Round post volume uses the column formula.
How much concrete for a 4×4 fence post
- Hole: 10 in diameter, 2 ft deep → π × (10 ÷ 24)² × 2 = 1.09 ft³.
- Post: a 4×4 is actually 3.5 × 3.5 in → (3.5 ÷ 12)² × 2 = 0.17 ft³.
- Concrete: 1.09 − 0.17 = 0.92 ft³.
- 80 lb bags: 0.92 ÷ 0.60 = 1.5, so 2 bags. 50 lb bags: 0.92 ÷ 0.375 = 2.5, so 3 bags.
A common rule of thumb is a hole about three times the post width, with one third to one half of the post buried . Frost depth and fence height can change that. Fast-setting post mixes often yield differently than standard concrete, so check the bag.
Concrete column calculator tips
- Many tube forms are sold in 12 ft lengths and cut to size. Measure the buried depth plus the part above grade.
- A wide footing under the column, often a square pad or a bell-shaped base, is extra volume. Add it in the concrete footing calculator.
- For a mix of columns, slabs, and stairs, use the main concrete calculator.
Volume only. Column size, rebar, and footing design come from your plans or an engineer.