What this calculator works out
This calculator applies the standard rule of burying at least a third of the above-ground fence height, adds an allowance for a drainage layer beneath the post, and returns the hole depth and the total post length to buy.
Fence failure is almost always a post problem rather than a panel problem. Posts that are too shallow lever out in high wind, and posts sitting in water rot at ground level.
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Before you dig
- Height is the fence above ground, not the post length. The calculator adds the buried depth for you.
- A minimum depth of 450 mm applies regardless of height, and the calculator enforces it.
- The extra allowance covers a layer of hardcore or gravel in the bottom of the hole for drainage. 100 mm is typical.
- Check for buried services before digging. Electricity, gas, water and telecoms all run at fence-post depth in places.
A post set in a sealed concrete plug sits in water that cannot drain. Putting 100 mm of hardcore in the bottom lets water escape, and sloping the top of the concrete away from the post stops water pooling around it. Those two details do more for post life than the depth does.
How this calculator works
A proportional depth with a floor:
Hole depth = fence height ÷ 3 + extra allowance
Never less than 0.45 m
Total post length = hole depth + fence heightThe one-third rule is a long-standing rule of thumb for domestic fencing rather than an engineered figure. Exposed sites, tall close-boarded fences and soft ground all warrant going deeper.
Wind load rises sharply with height and with how solid the fence is. A 1.8 m close-boarded fence carries far more load than a 1.8 m post-and-rail, and the posts should reflect that.
Worked example: a 1.8 m fence
Using the default figures — 1.8 m above ground with a 100 mm allowance:
- Hole depth: 1.8 ÷ 3 + 0.1 = 0.70 m
- Total post length: 2.5 m
2.5 m is not a standard stocked length everywhere — 2.4 m is far more common, which would give either a 0.6 m burial or a fence 100 mm lower. That is exactly the sort of thing worth discovering before ordering rather than on site. Where a 2.4 m post is what you can get, reducing the fence height slightly is usually better than reducing the depth.
Common mistakes
- Digging to a fixed 600 mm regardless of fence height. Taller fences need more.
- Sealing the post base in concrete with no drainage. Add hardcore beneath.
- Finishing the concrete flat or dished. Slope it away so water runs off.
- Digging without checking for services. Cables and pipes run at this depth.
- Assuming the boundary is where the old fence was. Check the deeds before moving anything.
Frequently asked questions
Concrete or postcrete?
Postcrete sets in minutes and is convenient for one or two posts, at a higher cost per post. Standard concrete is cheaper for a long run and gives more working time to plumb and align posts before it goes off. Both work; the choice is mainly about how many posts and how much time you have.
How far apart should posts be?
Panel fencing is dictated by the panel width, usually 1.83 m, so posts go at that spacing plus the post width. For close-boarded fencing built on site, 1.8 to 2.4 m is typical, with closer spacing on exposed sites or for taller fences.
Do I need planning permission for a fence?
Generally not up to 2 m in height, or 1 m where the fence adjoins a highway used by vehicles. Listed buildings, conservation areas and properties with specific planning conditions may differ, and the limits apply to the height above ground level on the higher side where levels differ.
Whose fence is it?
There is no reliable convention, despite the widespread belief about the left-hand side. Ownership is set out in the title deeds and the Land Registry plan, sometimes with a T-mark showing which side carries responsibility. Check before removing or replacing a boundary fence, and talk to the neighbour regardless.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — planning rules on fences, gates and boundary treatments
- Health and Safety Executive — avoiding underground services when digging
Sources are checked at publication and can change — how I choose and check references.