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Fence Calculator

Calculators · Added

The arithmetic that catches people is the post count, and it is the fence-post riddle itself: a run of four sections needs five posts. Everything else — rails, pickets, concrete — follows from that, so getting it wrong compounds through the whole order.

Units

Vertical boards fixed to horizontal rails between posts.

Centre to centre

Two for a low fence, three or more above about 1.5 m

Zero for a solid, close-boarded fence

How to use the fence calculator

  1. 1Enter the total run, the spacing you want between posts, and the finished height.
  2. 2Add any gates. Their width comes out of the fenced length and each one adds a pair of posts.
  3. 3For a picket fence, set the picket width and the gap between them.
  4. 4Read the materials list, and the concrete figure for the post holes.

Examples

A garden boundary

Input
30 m at 2.4 m spacing, 1.8 m high, one 1 m gate
Result
14 line posts, 2 gate posts, 13 sections at 2.23 m, 46 bags of concrete

The gate takes 1 m out of the run, and the spacing closes up so 29 m divides evenly.

The post-count trap

Input
Any run divided into 12 sections
Result
13 posts, not 12 — the last section needs a post at each end

About the fence calculator

Why the spacing is a range rather than a number

Most timber fencing is built at 1.8 to 2.4 m between post centres. Closer than that wastes posts; wider and the rails begin to sag under their own weight and the fence flexes in wind. Panel fencing has no choice at all, since the panel width fixes it.

The right end of the range depends on the rail. A deep, well-seasoned rail spans further than a thin one, and a fence carrying close-boarded infill catches far more wind than an open post-and-rail, which is why the same spacing behaves differently on two fences of the same height.

Concrete, and what it is actually doing

The concrete in a post hole is not glue. It is mass and bearing area: it spreads the overturning force from the fence into a much larger volume of soil than the post alone would touch. That is why a wider hole helps more than a deeper one up to a point, and why the volume calculated here subtracts the post from the hole.

It is also why a post set in concrete but standing in a puddle rots anyway. The usual detail is a layer of gravel below the post for drainage, with the concrete shaped to shed water away from the timber rather than forming a bowl around it.

Frequently asked questions

Why is there one more post than section?
Because every section needs a post at each end, and consecutive sections share one. Four sections have five boundaries between and around them. It is the classic fence-post problem and it is the single most common ordering error on a fence, because the mistake is invisible until the last section has nothing to fix to.
How deep should the posts go?
The trade rule of thumb is a third of the post's total length in the ground, with a minimum of about 600 mm, and that is what this uses. It is a guideline rather than a code requirement and it is not enough everywhere: where the ground freezes, the footing has to reach below the local frost line or the post will lift over a winter. That depth comes from your building department, not from a formula.
Why does the post spacing change from what I entered?
Because the run is divided into whole sections. If 30 m at 2.4 m spacing gives 12.5 sections, the tool rounds up to 13 and closes the spacing to 2.31 m rather than leaving a half-width bay at the end. Panel fencing cannot do this — the panel width is fixed — so that style is flagged separately.
Do I need permission?
Often, above a certain height. In the UK the usual threshold is 2 m, and 1 m where a fence adjoins a highway; most US jurisdictions require a permit above 6 or 7 feet and many have rules about boundary position and which side faces out. Wind load also starts to govern the post and footing design at that point. Check locally before ordering material.

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