How Much Concrete Does a Fence Post Need?
A typical 4×4 fence post in a 10 in hole, 36 in deep with 6 in of gravel underneath, takes about 1.15 cu ft of concrete: just under two 80 lb bags, or just over three 50 lb bags. Bigger posts need wider holes and more concrete.
How to work it out
- Hole area: π × (diameter ÷ 2)². For a 10 in hole, 3.1416 × 5² = 78.5 sq in.
- Subtract the post: a 4×4 is actually 3.5 × 3.5 in = 12.25 sq in. Concrete area: 66.3 sq in.
- Concrete depth: hole depth minus gravel, 36 − 6 = 30 in.
- Volume: 66.3 × 30 = 1,989 cu in. Divide by 1,728 to get cubic feet: 1.15 cu ft.
- Bags: 1.15 ÷ 0.60 (80 lb bag) = 1.9 bags.
Concrete per post: quick reference
All rows assume 6 in of gravel in the bottom of the hole and no extra allowance. "Buy" rounds up for filling each hole from its own bags.
| Post and hole | Concrete | 50 lb bags | 80 lb bags |
|---|---|---|---|
| 4×4 post (3.5 in), 10 in hole, 24 in deep | 0.69 cu ft | 1.8 (buy 2) | 1.2 (buy 2) |
| 4×4 post (3.5 in), 10 in hole, 30 in deep | 0.92 cu ft | 2.5 (buy 3) | 1.5 (buy 2) |
| 4×4 post (3.5 in), 10 in hole, 36 in deep | 1.15 cu ft | 3.1 (buy 4) | 1.9 (buy 2) |
| 4×4 post (3.5 in), 12 in hole, 36 in deep | 1.75 cu ft | 4.7 (buy 5) | 2.9 (buy 3) |
| 6×6 post (5.5 in), 16 in hole, 36 in deep | 2.97 cu ft | 7.9 (buy 8) | 4.9 (buy 5) |
| 6×6 post (5.5 in), 16 in hole, 42 in deep | 3.56 cu ft | 9.5 (buy 10) | 5.9 (buy 6) |
| Post and hole | Concrete | 20 kg bags |
|---|---|---|
| 75 mm post, 225 mm hole, 600 mm deep | 15 L | 1.6 (buy 2) |
| 100 mm post, 300 mm hole, 600 mm deep | 27 L | 2.9 (buy 3) |
| 100 mm post, 300 mm hole, 750 mm deep | 36 L | 3.9 (buy 4) |
| 100 mm post, 300 mm hole, 900 mm deep | 46 L | 4.9 (buy 5) |
| 125 mm post, 375 mm hole, 900 mm deep | 71 L | 7.6 (buy 8) |
Sizing the hole
Quikrete's guidance is a hole about three times the post's width, a depth of one-third to one-half of the post's height above ground, plus 6 in, and about 6 in of gravel in the bottom for drainage. For a 4×4 post standing 6 ft tall, that's a hole about 10–12 in wide and 30–42 in deep. Local frost depth and building rules can require deeper holes.
Common mistakes
- Using the post's nominal size. A 4×4 measures 3.5 in, a 6×6 measures 5.5 in.
- Not subtracting the post. In a narrow hole the post takes up a large share of the space, so ignoring it overbuys.
- Buying the pooled total when you'll pour each hole separately. Leftover part-bags add up across many holes.
- Bell-shaped holes. Holes that widen at the bottom take more concrete than a straight cylinder. Add 10% or more.
