How Much Concrete Do I Need for Fence Posts? (2026 Guide)
How much concrete for fence posts? A typical 4×4 post hole is 8–10 in wide × 24–36 in deep and takes roughly half a bag to a full bag of mix. Budget 0.8 bag of an 80-lb mix per post: 10 posts ≈ 8 bags of 80-lb or 10–11 of 60-lb.
How much concrete does one 4×4 post need?
Bag yield is what counts: a 60-lb bag makes about 0.45 cf of concrete and an 80-lb bag makes about 0.60 cf. Mix each bag with the water amount on the label — too much weakens the mix, too little leaves dry pockets.
A typical hole for a 4×4 post is 8–10 in wide and 24–36 in deep, which — once the post sits on a 6-in gravel bed — leaves roughly 0.4–0.6 cf of space to fill around it: about half a bag to a full bag of an 80-lb mix, compact holes at the low end and frost-line holes at the top.
For a whole fence, budget 0.8 bag of an 80-lb mix per post or one 60-lb bag per post for margin. Rule of thumb: one 80-lb bag covers about 1.3 typical posts, so 10 posts ≈ 8 bags.
Worked example: a 10-post fence
Say your run needs 10 posts dug 8–10 in wide and about 30 in deep. At the 0.8-bag rate: 10 × 0.8 = 8 bags of 80-lb mix, about 0.8 bag per post. On the 60-lb track, budget 1 bag per post — 10 bags plus one spare = 10–11 bags.
Need the post count first? The fence calculator counts posts for your run; the concrete calculator turns your exact hole size into bags.
Concrete bags for 4 to 12 posts
Whole-job math is simple: 80-lb bags ≈ posts × 0.8 and 60-lb bags ≈ posts, plus one spare. Figures below are rounded to whole bags for typical 8–10 in × 24–36 in holes on ordinary soil:
| Posts | 80-lb bags (about 0.8 per post) | 60-lb bags (1 per post) |
|---|---|---|
| 4 | 3 | 4 |
| 6 | 5 | 6 |
| 8 | 6–7 | 8 |
| 10 | 8 | 10 |
| 12 | 10 | 12 |
Counts assume ordinary, well-draining soil and typical 8–10 in × 24–36 in holes — planning estimates, not a substitute for measuring. Frost-line diggings, wide holes or soft ground push a row up; compact holes can come in lighter. Confirm local frost depth and coverage with your supplier.
Fence post concrete tips
- Go below the frost line. Set the base past your local frost line (often 30–48 in in cold states) and bury at least a third of the post's length; concrete can't stop frost heave in a hole that's too shallow.
- Settle about 6 in of gravel in the bottom. It drains water away from the post base and gives the concrete a firm bed, slowing rot and frost heave. Pack it level before setting the post.
- Fast-setting vs. regular mix. Fast-setting bagged mixes are made for posts — pour them into the hole, add the labeled water, and they hold a post without forms the same day. Regular mix costs less but cures over days, so the post stays braced longer.
- Brace before you pour, especially on slopes. Hold the post plumb with a temporary 2×4 brace and stakes until the mix sets; a leaning post can't be fixed once the concrete hardens.
Planning the rest of the yard? Pair this with how many fence pickets you need and, for other pours, concrete for a 10×10 slab.
FAQ
How many post holes does one 80-lb bag of concrete fill?
Roughly one to two regular 4×4 post holes. An 80-lb bag yields 0.60 cf, and a typical hole needs about 0.5 cf around the post — about 0.8 of the bag. One bag covers about 1.3 typical holes (10 posts ≈ 8 bags), or two compact 24-in holes at half a bag each.
How deep should a fence post hole be?
About one-third of the post's length should sit in the ground — roughly 32 in for an 8-ft post — and the base must reach below your local frost line where winters freeze (often 30–48 in). Confirm your region's frost depth before digging.
What size hole do I need for a 4×4 fence post?
Dig it about 8–10 in wide — two to three times the post width — and 24–36 in deep, with about 6 in of gravel in the bottom. Go deeper where the local frost line requires it.
Need a different size?
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