How Much Concrete Per Fence Post?
The short answer: two 50 lb bags per line or end post, three per gate post. The long answer — holes, depth, frost, and the mistakes that make posts heave or wobble — is what separates an estimate that holds from one that eats warranty callbacks.
Bags per post
| Post type | 50 lb bags | Mixed volume | Why |
|---|---|---|---|
| Line post | 2 | ≈ 0.6 ft³ | Carries vertical load only |
| End post | 2 | ≈ 0.6 ft³ | Holds the run tension |
| Corner post | 2 | ≈ 0.6 ft³ | Tension from two directions |
| Gate post | 3 | ≈ 0.9 ft³ | Carries the swinging gate |
At $5–$7 per bag, concrete is the cheapest part of the fence — undershooting it is the most expensive mistake per dollar saved. A gate post with two bags is the post that leans within a season and then takes the latch and the gate with it.
Hole sizing
The hole is three times the post width and deep enough to reach below frost:
- 4×4 post → 12 in diameter hole
- 6×6 post → 14 in diameter hole
- Depth: one-third of the post length minimum, below the frost line in practice — 36–42 in in southern Canada and the northern US, deeper in the prairies and far north.
Check local code for both depth and whether concrete is required at all (some municipalities accept packed gravel for line posts; almost none for gates).
Frost and the bell
Frost doesn't lift posts because the ground is cold — it lifts them because wet soil freezes, expands, and grabs the concrete. Three defences:
- Go below the frost line. The bottom of the hole below the freeze zone is the primary defence.
- Bell the concrete. A wider pad at the bottom of the pour means expanding soil pushes against the bell's bottom edge and can't push the post up. Cheap insurance in heave country.
- Slope concrete away from the post at grade so water runs off instead of pooling at the collar.
Mixing and setting
- Mix stiff, not soupy — a dry mix (gravel-wet) sets stronger and props the post better than a wet pour.
- Set the post on gravel or a couple of inches of concrete at the hole bottom; don't let the post sit on dirt.
- Brace or check plumb twice: once at half-fill, once at top. You can't fix a leaning pour after it kicks.
- In cold weather use an accelerant, and don't set posts when the ground is frozen — the pour will heave before it cures.
The math on a real job
100 ft wood fence, 8 ft spacing, one gate:
- 14 line/end posts × 2 = 28 bags
- 2 gate posts × 3 = 6 bags
- Total: 34 bags ≈ 5.5 ft³ of concrete, roughly $190 at starter prices.
The fence material calculator runs this per post type automatically — flip the gate count and the concrete line moves with it. And when the layout has real geometry, FenceTracer's takeoff attaches the bags to the classified posts (line/end/corner/gate) so the crew loads exactly what the quote promised.
FenceTracer
Concrete counts that match the quote
FenceTracer attaches 2 bags per line post and 3 per gate post to the takeoff automatically — the crew loads what the customer paid for. Free for 3 months.Frequently asked questions
Is 2 bags of concrete enough for a 4x4 fence post?
For a line post, yes — two 50 lb bags fill a 12 in hole to frost depth. Gate posts get three bags because they carry the gate's swing load.
How deep should a 6-foot fence post be?
At least 24–30 in (one-third of the 8 ft post), and below frost line where frost runs deeper — 36–42 in across much of Canada. A 6 ft fence with an 8 ft post set 2 ft deep is fine in frost-free ground only.
Can you set fence posts without concrete?
Packed gravel works for some line posts where code allows, but gate posts, corners, and terminals should be concreted — they carry tension and swing loads that gravel can't hold.
