Fencing & decking
Fence Post Calculator
Works out post count, hole depth and concrete, where depth is set by frost rather than by the fence.
Result
The one-third rule is a starting point. Frost depth, soil type and wind exposure all override it, and in cold regions the hole must reach below the frost line or the fence lifts every winter.
Get a Free Business AuditHow the calculation works
Posts are the run divided by spacing plus one. Depth follows the common rule of about a third of the above ground height with a practical minimum, which is what resists the leverage the fence puts on the post. Concrete per hole is the hole volume less the volume the post itself occupies, since the post displaces concrete. The rule of thumb on depth is a starting point rather than the governing figure: in a cold climate the frost line is what actually matters, because a post set above it will be pushed up as the ground freezes and will keep moving every winter.
What moves the number
- Local frost depth, which overrides the height-based rule in cold climates.
- Above ground height, since taller fences exert more leverage and need more depth.
- Soil type, as sand, clay and rock each behave differently and rock may limit depth.
- Whether the post is a line, corner or gate post, which changes both size and depth.
Where this one goes wrong
Call the utility locate service before digging, every time. It is free, it is a legal requirement in most places, and hitting a gas line or a buried electrical service with a post auger is a serious injury and a large liability. Mark and wait for the locate rather than assuming a residential back garden has nothing in it. On the technical side, set posts below the frost line in any climate that freezes, because the height rule alone will leave posts heaving. And do not pour concrete into a hole with no drainage at the bottom in a wet soil: water collecting around the post base is what rots timber posts, so gravel at the base and a crown at the top to shed water are both worth doing.
Before you quote
- Call the utility locate service and wait for the marks before digging.
- Set depth below the local frost line, not just to the height rule.
- Put gravel at the hole base and crown the concrete to shed water.
Where the rules come from
Questions
About this calculator
How deep should fence posts be?
The common rule is about a third of the above ground height with a practical minimum, but in a climate that freezes the frost line governs. A post set above the frost line will be lifted as the ground freezes and will keep moving each winter, which no amount of concrete fixes.
Do I really need to call before digging for fence posts?
Yes. It is free, it is required in most places, and buried gas and electrical services are present in ordinary residential gardens. Hitting one with an auger is a serious injury risk and a large liability, and the locate takes a few days to arrange rather than weeks.
How much concrete per fence post?
The hole volume less the volume of the post, since the post displaces concrete. A wider or deeper hole obviously takes more, and gate and corner posts usually need both. Working it out per hole and multiplying is more accurate than a bags-per-post rule.
Should the concrete be crowned at the top?
Yes, sloping away from the post so water runs off rather than sitting against it. Water collecting at the post base is the main cause of rot in timber posts, and a flat or dished top holds it there. Gravel at the base of the hole helps for the same reason.
The formula
posts = length / spacing + 1; depth = height / 3, minimum 2 ft; concrete = hole volume less the post
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How to use this calculator
- Fence length (ft) (ft). Enter your own figure; the page starts at 150 ft.
- Post spacing (ft) (ft). Enter your own figure; the page starts at 8 ft.
- Fence height (ft) (ft). Enter your own figure; the page starts at 6 ft.
- Hole diameter (in) (in). Enter your own figure; the page starts at 10 in.
- Corner and end posts. Enter your own figure; the page starts at 4.
- Cost per post ($). Enter your own figure; the page starts at $18.
The result updates as you type, so you can try a figure, see what it does to total posts, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Fence length (ft) | 150 ft |
| Post spacing (ft) | 8 ft |
| Fence height (ft) | 6 ft |
| Hole diameter (in) | 10 in |
| Corner and end posts | 4 |
| Cost per post | $18 |
What comes out
| Result | Value |
|---|---|
| Total posts main result | 20 |
| Hole depth | 2.0 |
| Post length to buy | 8.0 |
| 60 lb bags of concrete | 42 |
| Post cost | $360 |
20 posts, each set 2.0 feet deep, so buy 8.0 foot posts. Allow about 42 sixty-pound bags of concrete.
How sensitive the answer is
Holding everything else steady and changing only cost per post, this is what happens to total posts. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per post ($) | Total posts |
|---|---|
| $9 | 20 |
| $14 | 20 |
| $18 example above | 20 |
| $27 | 20 |
| $36 | 20 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Fence length (ft) (ft)
- Measured in ft, starting at 150 ft.
- Post spacing (ft) (ft)
- Measured in ft, between 3 and 16, starting at 8 ft.
- Fence height (ft) (ft)
- Measured in ft, between 0 and 12, starting at 6 ft.
- Hole diameter (in) (in)
- Measured in in, between 0 and 36, starting at 10 in.
- Corner and end posts
- A number, between 0 and 40, starting at 4.
- Cost per post ($)
- A money amount, starting at $18.
What the results tell you
| Result | What it is |
|---|---|
| Total posts | The headline result. A number. |
| Hole depth | A decimal figure. |
| Post length to buy | A decimal figure. |
| 60 lb bags of concrete | A number. |
| Post cost | A money figure. |
Keep going
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