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Decks & Fences

Fence Post Concrete Calculator

Setting fence posts takes more concrete than most people expect — a single 10 inch hole 2½ feet deep swallows roughly three 50 lb bags. Enter your post count, hole size, and post size to get the concrete volume per hole and the number of fast-setting bags to buy, with the buried post volume already subtracted.

Reviewed by The CostToUpgrade Estimating Desk Last reviewed How we estimate

Your project

Layout

Include gate posts and corner posts — they use the same or larger holes.

Hole

Rule of thumb: about 3× the post width, so 10–12 in for a 4x4.

Aim for 1/3 of the post height and always below your local frost line.

Wood posts are measured actual, not nominal — a 4x4 is really 3.5 in square.

Pricing

Fast-setting post mix typically runs $5–$9 per 50 lb bag.

Estimate

62bags

50 lb bags needed · Fast-setting mix, ≈ 0.375 ft³ per bag

Concrete per hole
1.151ft³
Total concrete
23.02ft³
Estimated concrete cost
$403.00

Estimates are guidance only and exclude tax, labor, and site conditions unless stated. Confirm with a local pro.

How it's calculated

No black boxes — here is the exact math behind your estimate, so you can check it or adapt it for your own quote.

  1. 1Hole volume (ft³) = π × (hole diameter ÷ 24)² × hole depth
  2. 2Concrete per hole = hole volume − buried post volume (actual post size)
  3. 3Total concrete = concrete per hole × number of posts
  4. 4Bags (50 lb) = ⌈ total ft³ ÷ 0.375 ⌉
  5. 5Cost = bags × price per bag

Worked example

Twenty 4x4 wood posts set in 10 inch holes dug 2.5 feet deep with fast-setting mix.

Inputs

Number of posts
20 posts
Hole diameter
10 in
Hole depth
2.5 ft
Post size
3.5
Price per 50 lb bag
6.5 $/bag

Result

50 lb bags needed
62 bags
Concrete per hole
1.151 ft³
Total concrete
23.02 ft³
Estimated concrete cost
$403.00

Tools & materials for this project

As an Amazon Associate we may earn from qualifying purchases, at no extra cost to you.

  • Fast-setting post concrete Sets in under an hour with no mixing required.
    View on Amazon
  • Post level Straps to the post so it stays plumb on two faces while the mix sets.
    View on Amazon
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Frequently asked questions

How many bags of concrete per fence post?
A 4x4 post in a 10 inch hole 2.5 feet deep needs about 1.15 cubic feet of concrete once the post itself is subtracted — just over three 50 lb bags of fast-setting mix, or two 80 lb bags. Widen the hole to 12 inches and it jumps to about 1.75 cubic feet, or five 50 lb bags. Corner, end, and gate posts usually get a wider or deeper hole, so buy a few extra bags rather than running short mid-run.
How deep should a fence post be set?
The standard rule is one third of the post's above-ground height, and never shallower than your local frost line. A 6 ft fence therefore wants about 2 to 2.5 feet of buried post, which means an 8 or 9 ft post. In cold climates the frost line governs: 36 to 48 inches is common across the northern US, while much of the South only requires 12 to 24 inches. Check with your building department — frost depth is set locally.
How wide should the post hole be?
Dig the hole about three times the post width. That means 10 to 12 inches for a 3.5 inch 4x4 and 16 to 18 inches for a 6x6. Narrower holes leave a thin concrete collar that cracks; much wider holes just burn concrete. Keep the hole straight-sided or slightly belled at the bottom, not cone-shaped at the top, so frost cannot grab the collar and heave the post out over the winter.
Can I dry-pour concrete into the hole?
Fast-setting post mixes are formulated for it: pour the dry mix to about 3 to 4 inches below grade, add the water volume printed on the bag (roughly 1 gallon per 50 lb bag), and it sets in 20 to 40 minutes. Do not skip the water and rely on rain or ground moisture — the cure is uneven and the collar ends up weak and crumbly. For standard concrete mix, mix it in a tub or wheelbarrow before it goes in the hole.
Should I put gravel at the bottom of the hole?
Yes for wood posts. Four to six inches of compacted gravel under the post lets water drain away from the end grain instead of pooling in a concrete cup, which is where posts usually rot first. Subtract that gravel depth from your concrete depth when estimating. Then slope the top of the concrete collar away from the post and keep it an inch above grade so surface water sheds instead of sitting against the wood.

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