Bags per hole for fence, deck and mailbox posts — with the post itself subtracted, which most calculators forget.
Post Hole Concrete Calculator
Inputs
Actual, not nominal — a 4×4 measures 3.5 in and a 6×6 measures 5.5 in.
Drains water away from the post end. Set 0 if you are pouring to the bottom.
Results
Concrete needed
30 × 50 lb bags
per hole = π × (dia ÷ 24)² × pour depth − post² × pour depth
Calculated with BuildCalculators.com
Calculated with BuildCalculators.com · buildcalculators.com/post-hole-concrete-calculator
Estimates are for planning only — confirm quantities with your supplier or contractor before ordering.
Setting a post is a small pour repeated a lot of times, which is what makes the per-hole number the one that matters. Get it wrong by half a bag and a hundred-foot fence is six bags out — either a second trip to the store partway down the line, or a pallet of leftovers.
The arithmetic is a cylinder with a square hole in the middle of it. Most calculators skip the second part, and the post is not a rounding error: a 4×4 in a 10 inch hole 26 inches deep displaces 0.18 cubic feet, which is very nearly half a 50 lb bag on every single post.
hole depth − gravel bedπ × (diameter ÷ 24)² × pour depth(post width ÷ 12)² × pour depthhole − postfloor(length ÷ spacing) + 1The standard sizing rule is that the hole is three times the post width in diameter and a third of the post's total length goes into the ground. An 8 ft 4×4 therefore wants a hole about 10½ inches across and 32 inches deep, leaving 5 ft 4 in of fence above grade. It is a rule of thumb rather than a code requirement, but every fence supplier sizes their posts around it, which is why a "6 ft fence post" is sold at 8 or 9 feet.
Where the frost line is deeper than a third of the post, the frost line wins and you buy longer posts. A post set above frost depth gets jacked upward every winter, and because it does not come back down evenly, the whole fence line goes out of true within a few seasons. This is the single most common reason a fence that was straight in year one looks wavy in year four.
Four to six inches of compacted gravel under the post lets water drain away from the end grain instead of standing against it. Concrete poured to the very bottom makes a cup: water gets in around the post — it always does — and then has nowhere to go. The gravel bed also reduces the concrete slightly, which the calculator accounts for by pouring only above it.
A 100 ft run at 8 ft spacing has 12 gaps and therefore 13 posts. Dividing the length by the spacing gives you the gaps, not the posts, and forgetting the plus-one is the oldest off-by-one error there is. Corner, end and gate posts also want to be one size up with a wider hole, because they carry sideways load that line posts never see.
Fast-setting mix goes into the hole dry and gets water poured on top; standard mix is mixed to a slump first and then placed. Both hold a post fine. What does not work is mixing the two methods along one fence — they cure at different rates, so the posts you can straighten and the posts you cannot are decided by which bag you happened to open. Pick one and use it for the whole run.
Finish the concrete above grade and slope it away from the post on every side. A flat or dished top holds a ring of water exactly where the post enters the concrete, and that collar is where posts rot from — not from the buried end, which people expect, but from the waterline they built themselves.
10 in holes, 30 in deep, 4 in gravel under each post.
39 bags of 50 lb — three per hole
Ignoring the post displacement would add about half a bag per hole here. Across 13 posts that is six wasted bags, and across a big job it is a pallet.
An 8 ft 4×4 for a 5 ft fence — what hole does it want?
A 10½ in hole, 32 in deep
If your frost line is deeper than 32 inches, that governs instead and you need a 9 or 10 ft post. Check the local figure before buying the posts, not after.
16 in holes, 36 in deep, no gravel — poured to the bottom.
36 bags of 80 lb
At 21 ft³ total this is close to a cubic yard. Six holes is still bag territory, but a couple more and a short-load delivery starts to win.
Bags rounded up per hole — a part bag does not carry to the next one.
| Hole | Depth | Concrete | 50 lb bags | 60 lb bags |
|---|---|---|---|---|
| 8 in | 24 in | 0.44 ft³ | 2 | 1 |
| 10 in | 24 in | 0.77 ft³ | 3 | 2 |
| 10 in | 30 in | 1.00 ft³ | 3 | 3 |
| 12 in | 30 in | 1.52 ft³ | 5 | 4 |
| 12 in | 36 in | 1.87 ft³ | 5 | 5 |
| 16 in | 36 in | 3.50 ft³ | 10 | 8 |
Three times the post width across, one third of its length buried.
| Post | Hole diameter | Hole depth | Above ground |
|---|---|---|---|
| 4×4, 8 ft | 10.5 in | 32 in | 5.33 ft |
| 4×4, 9 ft | 10.5 in | 36 in | 6.00 ft |
| 6×6, 10 ft | 16.5 in | 40 in | 6.67 ft |
| 6×6, 12 ft | 16.5 in | 48 in | 8.00 ft |
Concrete is the cheap part. A 50 lb bag of fast-setting mix runs $5–$8 in 2026, so three bags a hole is $15–$24 per post — against $25–$60 for the post itself in pressure-treated pine, and considerably more in cedar. A 100 ft fence is therefore roughly $200–$300 of concrete inside a $2,000–$4,000 job.
Digging. A two-person auger rents for $60–$100 a day and handles clean soil; hit clay, roots or rock and you are hand-digging or renting a skid-steer auger at $300–$500. On a fence line that crosses a tree root zone, the digging can cost more than every other material combined.
Fast-setting mix costs 10–20% more per bag and saves the mixing entirely — you pour it dry, add water, and it holds the post in under an hour. For under about 20 posts that convenience is worth the premium. Past that, standard mix and a mixer is meaningfully cheaper, and you are already set up for a day of work either way.
On depth, and on the corner posts. A shallow hole costs a bag less today and a leaning fence in three years. Corner, end and gate posts take all the tension in the run — going one size up and one hole size wider at those five or six locations adds very little to the total and is the difference between a fence that stays square and one that does not.
The concrete fills the hole minus the post. On a 4×4 in a 10 inch hole that is over a third of a bag per post — small once, and a pallet of surplus across a big fence line.
That gives you the gaps. The posts are one more. A 100 ft run at 8 ft spacing is 13 posts, not 12, and the one you forget is the one at the end.
Frost jacks them up unevenly, and they do not settle back. The rule of thumb depth is a minimum, not a permission — where frost goes deeper, the hole follows it and the posts get longer.
It makes a cup that holds water against the end of the post. Four to six inches of compacted gravel underneath gives the water somewhere to go.
Water pools at the collar, which is where posts actually rot from. Crown it above grade and slope it away on every side.
They cure at different rates, so some posts can still be adjusted and others cannot. Pick one method for the whole fence.
For the common case — a 4×4 in a 10 inch hole 30 inches deep on 4 inches of gravel — about 1.0 cubic foot per hole, which is 3 bags of 50 lb or 3 of 60 lb. Wider or deeper holes climb fast: a 12 inch hole 36 inches deep needs 5 bags of 50 lb.
Three times the post width across, and a third of the post length in the ground. An 8 ft 4×4 wants a hole about 10½ inches across and 32 inches deep, leaving 5 ft 4 in above grade. Where the frost line is deeper than that, the frost line governs and you need a longer post.
Yes, and it matters more than people expect. A 4×4 in a 10 inch hole poured 26 inches deep displaces 0.18 cubic feet — roughly half a 50 lb bag on every post. Across a 13-post fence that is six bags you would otherwise buy and not use.
A third of the total post length, or below the local frost line, whichever is deeper. A post set above frost depth gets jacked upward every winter and does not settle back evenly, which is why fence lines that were straight in year one go wavy by year four.
6 to 8 feet is standard, closer on tall or solid-panel fences that catch wind. Remember the count: a 100 ft run at 8 ft spacing has 12 gaps and therefore 13 posts.
Four to six inches of compacted gravel, yes. Concrete poured to the bottom makes a cup that holds water against the end grain. The gravel gives it somewhere to drain, and the calculator pours only above it.
It is faster and costs 10–20% more; you pour it in dry and add water. Both fast-set and standard mix hold a post fine. What causes trouble is using both along one fence, because they cure at different rates and you lose the ability to adjust some posts and not others.
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This calculator uses published constants from named industry sources, each asserted in an automated test that runs on every build. The formula is printed with the result above, and any correction we make is logged on the changelog.
It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.