Concrete Calculator for Fence Posts
{posts} posts in {holeDia}-inch holes {holeDepth} inches deep need about {bags} bags of {bag}-lb concrete — {bagsPerPost} per post, or {cubicYards} cubic yards if you order ready-mix.
Bags per post for common hole sizes
| Hole diameter | Hole depth | Post | Concrete per post | 50-lb bags | 60-lb bags | 80-lb bags |
|---|---|---|---|---|---|---|
| 8 in | 24 in | 4×4 | 0.58 cf | 2 | 2 | 1 |
| 10 in | 24 in | 4×4 | 1.01 cf | 3 | 3 | 2 |
| 10 in | 30 in | 4×4 | 1.27 cf | 4 | 3 | 3 |
| 12 in | 24 in | 4×4 | 1.54 cf | 5 | 4 | 3 |
| 12 in | 30 in | 4×4 | 1.93 cf | 6 | 5 | 4 |
| 12 in | 36 in | 4×4 | 2.31 cf | 7 | 6 | 4 |
| 16 in | 30 in | 6×6 | 3.26 cf | 9 | 8 | 6 |
| 16 in | 36 in | 6×6 | 3.91 cf | 11 | 9 | 7 |
| 18 in | 36 in | 6×6 | 5.14 cf | 14 | 12 | 9 |
| 18 in | 42 in | 6×6 | 5.99 cf | 16 | 14 | 10 |
Includes 10% extra and subtracts the post's actual dimensions. Multiply by your post count.
How the math works
Each hole is a cylinder. The post sitting in it is a rectangular prism you don't need to fill. Subtract one from the other, convert to cubic feet, add a little for spillage and uneven holes, then divide by how much concrete one bag makes.
post = post width × post width × depth
concrete per post = (hole − post) ÷ 1728 → cubic feet
bags = (concrete per post × posts × 1.10) ÷ bag yield
Bag yields used here are the manufacturer figures for placed concrete: 0.375 cubic feet from a 50-lb bag, 0.45 from a 60-lb bag, 0.60 from an 80-lb bag. A 4×4 post is actually 3.5 inches square and a 6×6 is 5.5 inches, and the calculator uses those real sizes.
The 1/3 rule is a floor, not an answer
Burying a third of the post's total length is a rough proxy for lateral stability under modest load in average soil. It doesn't account for frost, which lifts posts vertically regardless of how deep a third works out to; it doesn't account for wind, which is the dominant load on a solid privacy fence and has nothing to do with post length; and it doesn't account for soil type, which determines whether the buried portion can resist lateral force at all. Three conditions can each override the 1/3 rule on their own.
Override 1 — frost
The post base needs to sit below the local frost line, on the same principle as any footing (see the frost depth table on the footing calculator). This is where the 1/3 rule most often falls short: a 6-foot fence typically uses an 8-foot post, and a third of that is 32 inches — which is 10 inches short in a 42-inch frost zone.
Even a footing that reaches below frost depth isn't fully protected — soil freezing against the sides of the post (adfreeze) can lift it by friction alone. This is made worse by a common finishing habit: crowning the concrete at the surface, wider than the hole below it, to shed water. When surface soil freezes and grips that flare, it lifts the whole post, and the gap left underneath fills with soil — so the post doesn't fully settle back down. Repeat that over several winters and the post visibly ratchets upward.
The fix is shape, not more depth: keep the collar the same width or narrower at the top, and make sure the bottom is below frost. A footing that's wider at the bottom (belled) resists uplift; one that flares at the top invites it — the same principle covered on the sonotube page.
Override 2 — wind
A solid privacy fence acts like a sail; a picket fence with gaps doesn't. The same post depth that's adequate for one can be inadequate for the other, because wind load scales with the panel area each post is carrying — height times post spacing — and that load acts over the fence's full height, creating a large overturning moment right at grade. Gate posts carry both wind and the cantilevered weight of the gate itself, cycled every time it opens, and corner or end posts carry unbalanced tension from the run — which is why fences rarely fail in the middle of a straight run and consistently fail at gates, corners, and ends. Oversize those specifically.
Override 3 — soil
Dense clay gives good lateral resistance, and it's where the 1/3 rule holds up best. Sandy or loose granular soil gives poor lateral resistance — the soil tends to flow around the footing rather than resist it, which calls for more depth and more diameter, not just more depth. Expansive clay moves seasonally with moisture independent of frost, saturated soil loses both bearing and lateral capacity, and fill soil is unpredictable and usually poor — get below it if you can. These are the same soil bearing categories used on the footing calculator.
Before you buy
- Round up to whole bags per post, not per project. If each post needs 3.2 bags, buy 4 per post; you can't split a bag across holes cleanly.
- Fast-setting mix costs a little more per bag but lets you set posts and hang rails the same day. The bag count is identical.
- Put 3 to 6 inches of gravel in the bottom of the hole before the post for drainage. That reduces the concrete slightly — enter a shallower depth if you do.
- Above about 30 posts you're in the range where a small ready-mix delivery can beat bags on price. Use the cubic-yard figure above and check concrete cost per yard.
- Keep the concrete an inch or two below grade and slope it away from the post, or use a dome above grade, so water doesn't pool at the wood.
Common questions
How many bags of concrete per fence post?
For a 4×4 post in a 12-inch hole 30 inches deep, plan on four 80-lb bags, five 60-lb bags or six 50-lb bags. A 6×6 post in a 16-inch hole 36 inches deep needs about seven 80-lb bags.
How deep should a fence post hole be?
A common rule is one-third of the post's above-ground height, with a minimum of 24 inches, and below the frost line in cold climates. A 6-foot fence usually means a 30 to 36 inch hole.
How wide should the hole be?
About three times the post width: a 12-inch hole for a 4×4, a 16 to 18 inch hole for a 6×6. Wider holes hold more concrete and resist wind loads better.
Can I use fast-setting concrete without mixing?
Yes. Fast-setting mixes are designed to be poured dry into the hole and then watered. The bag count is the same as standard mix; use the same yields.
Should I subtract the post from the hole volume?
Yes, the post takes up space. This calculator subtracts the post's cross-section from the hole so you're not buying concrete for wood that's already there. Choose "None" under post size if you want the full hole volume.