Post hole concrete calculator
Setting a fence? Enter hole size, depth and how many posts — get total concrete volume and exact bag counts for 50 lb or 80 lb bags.
Result
Post concrete
How much concrete per fence post?
Post holes are cylinders minus the post itself:
Formula
ft³ per post = π × (hole Ø ÷ 2)² × depth ÷ 1728 − post volume
total = ft³ per post × number of posts · bags = ft³ ÷ bag yield
Example — privacy fence: 10-inch auger, 30-inch holes, twelve posts: each hole is 1.36 ft³ − 0.27 (4×4 in the ground 30 in) ≈ 1.09 ft³ → 13.1 ft³ ≈ 35×50 lb bags (or 22×80 lb).
Hole size rules that matter
| Post | Hole diameter | Typical depth |
|---|---|---|
| T-post / garden stakes | no concrete | tamp soil/gravel |
| 4×4 wood, 6 ft fence | 10 in | 24–30 in |
| 4×4 wood, 8 ft privacy | 10–12 in | 30–36 in (frost line!) |
| 6×6 gate / corner posts | 12–14 in | 36–48 in |
- Depth: bury ⅓ to ½ of the above-ground height, and at least to your local frost line (US frost lines run from 12 in in the South to 60+ in in Minnesota — call your building department).
- Drainage: 6 inches of gravel at the bottom of each hole keeps water from pooling against the post foot.
- Fast-setting mix: designed to pour dry into the hole and hose in — sets in 20–40 minutes, same yields as regular mix.
Frequently asked questions
How many bags of concrete per fence post?
A 4×4 in a 10 in hole 30 in deep takes about 3×50 lb bags (or 2×80 lb) of concrete after subtracting the post. Multiply by your post count — the calculator does exactly this.
Can you set fence posts with dry concrete mix?
Yes — fast-setting mixes are made for it: pour dry, add water, brace. Regular mixes should be pre-mixed wet for full strength.
How deep should a fence post be?
Rule of thumb: ⅓ to ½ of above-ground height, and never above the local frost line — heave will tilt shallow posts in one winter.
Do I need gravel under fence posts?
6 inches at the bottom is best practice — it drains water away from the buried end and dramatically slows rot on wood posts.
How many posts is a bag-a-post fence?
If each hole takes ~1 ft³, one 50 lb bag covers a third of a post. Budget 2–4 bags per post depending on hole size — or ~30 bags per 10 posts of privacy fence.