Bags per post at a glance
Each figure includes 10% extra and is rounded up to whole bags when you buy. The 50 lb column is fast-setting mix (0.375 cu ft per bag).
| Post | Hole | 80 lb | 60 lb | 50 lb fast-set |
|---|---|---|---|---|
| 4×4 | 10" × 24" deep | 1.7 | 2.3 | 2.7 |
| 4×4 | 10" × 30" deep | 2.1 | 2.8 | 3.4 |
| 4×4 | 12" × 36" deep | 3.9 | 5.1 | 6.2 |
| 6×6 | 12" × 36" deep | 3.2 | 4.2 | 5.1 |
| 6×6 | 16" × 42" deep | 7.6 | 10.2 | 12.2 |
| 6×6 | 18" × 48" deep | 11.4 | 15.2 | 18.3 |
For a whole fence, multiply by the number of posts and round up once at the end. Rounding up per post over-buys. The calculator above does it the right way.
Why the hole shape matters
Post-hole diggers and augers rarely cut a perfect cylinder. Holes flare at the top and bell at the bottom. That's the main reason fence jobs run short. If you're digging by hand in rocky soil, set the extra to 15%.
Common questions
How many bags of concrete per fence post?
For a 4×4 post in a 10" hole 30" deep, plan on about 2 × 80 lb bags, 3 × 60 lb bags, or 3–4 × 50 lb fast-setting bags per post. A 6×6 in a 12" hole 36" deep takes about 3–4 × 80 lb bags.
How deep should a fence post hole be?
A common rule is ⅓ of the post's above-ground height, so 24–30" for a 6 ft fence and 36" or more for 8 ft. In cold climates the footing must go below the frost line. Check your local code.
How wide should a post hole be?
About 3 times the post's width: 10–12" for a 4×4 and 12–16" for a 6×6. Gate posts and posts on tall privacy fences usually go a size bigger.
Fast-setting or regular concrete for fence posts?
Fast-setting mix (such as Quikrete Fast-Setting, 50 lb, about 0.375 cu ft) is poured dry into a hole with water and needs no mixing. It's set in about 20–40 minutes. Regular mix costs less per cubic foot and is often recommended for gate posts. Either works if you use enough.
Should I put gravel at the bottom of a post hole?
Many fence builders put 3–4" of gravel in the bottom so water drains away from the end grain of wood posts. If you do, subtract that from the hole depth here.
Prices last checked October 2026. Quantities only. Follow your local code for frost depth.