Worked example
Ten 4x4 posts in 12 in holes, 36 in deep, with 6 in of gravel and a 4 in soil cap, using QUIKRETE Fast-Setting 60 lb: concrete height = 36 − 6 − 4 = 26 in. Ring area = π × 6² − 3.5² = 113.1 − 12.25 = 100.85 in². Per post = 100.85 × 26 = 2,622 in³ ÷ 1,728 = 1.52 cu ft, or 3.4 bags. For the job: 10 × 1.517 ÷ 0.45 = 33.7, so 34 bags (2,040 lb, about 0.56 cu yd).
Bags per post (generated from the calculator’s formula)
4x4 post subtracted at 3.5 in, 6 in gravel, 4 in soil cap, round hole, no extra allowance.
| Hole Ø | Hole depth | cu ft / post | 50 lb fast (0.375) | 60 lb fast (0.45) | 80 lb mix (0.60) |
|---|---|---|---|---|---|
| 10 in | 24 in | 0.54 | 1.4 | 1.2 | 0.9 |
| 10 in | 30 in | 0.77 | 2.0 | 1.7 | 1.3 |
| 10 in | 36 in | 1.00 | 2.7 | 2.2 | 1.7 |
| 12 in | 24 in | 0.82 | 2.2 | 1.8 | 1.4 |
| 12 in | 30 in | 1.17 | 3.1 | 2.6 | 1.9 |
| 12 in | 36 in | 1.52 | 4.0 | 3.4 | 2.5 |
| 12 in | 42 in | 1.87 | 5.0 | 4.2 | 3.1 |
Checked against the manufacturer: filled to grade with no gravel or cap, a 10 in × 24 in 4x4 hole is 0.92 cu ft, or 2.4 bags at Sakrete’s 0.38 cu ft yield. Sakrete publishes 2.5 bags for that hole.
Common mistakes
- Using the hole volume without subtracting the post.
- Filling to the top. The method stops 3–4 in below grade and finishes with soil sloped away from the post.
- Using an 80 lb bag yield (0.60 cu ft) for a 50 lb fast-setting bag (0.375 cu ft).
- Rounding each hole up when you mix a fast-setting bag across holes. The job total rounds once.
Frequently asked questions
How many bags of concrete do I need per fence post?
A 4x4 post in a 10 in × 30 in hole takes about 2 bags of 50 lb fast-setting concrete (0.77 cu ft ÷ 0.375 = 2.0) with a 6 in gravel base and 4 in soil cap. In a 12 in hole of the same depth it takes about 3 (1.17 cu ft ÷ 0.375 = 3.1).
Should I subtract the post from the hole volume?
Yes. Concrete only fills the ring around the post. A 4x4 is 3.5 × 3.5 in actual, or 12.25 in² that would otherwise be counted as concrete.
Do I need gravel under a fence post?
QUIKRETE's Setting Posts method puts about 6 in of compacted gravel under the post for drainage and finishes 3–4 in below grade with soil. Both are subtracted from the concrete column.
Fast-setting or regular concrete mix?
Fast-setting mix is poured dry into a water-filled hole and is common for fence posts. Regular mix is mixed first and is often mixed hole by hole, which is why the calculator also shows a per-hole rounded bag count.
Why do bag yields differ?
Yield is published per product: QUIKRETE Fast-Setting makes 0.375 cu ft per 50 lb bag and 0.45 per 60 lb; Sakrete Fast Setting 0.38 per 50 lb; QUIKRETE Concrete Mix 0.60 per 80 lb.