Fence Posts, Panels & Rails Calculator

Understanding the fence calculator — concept and principle

This calculator estimates the posts, panels (or bays), rails and post concrete needed for a straight fence. Enter the total fence length, the panel or bay width, and the number of horizontal rails per bay. It is used to price a garden or boundary fence and to order materials before digging the first hole.

A straight run always has one more post than bays: 17 bays, 18 posts.

How it is calculated

Bays (panels) = ⌈fence length ÷ panel width⌉.
Posts = bays + 1.
Rails = bays × rails per bay.
Post concrete = posts × 2 bags of 25 kg.

Lengths are in metres. Rounding up means the last bay may be shorter than the others and will need a panel cut to size.

The principle

This is the classic fencepost problem: a straight line divided into n sections needs n + 1 posts, because every section needs a post at each end and neighbouring sections share one. Rails span between posts, so each bay gets its own set. Concrete scales with the number of holes; two 25 kg bags per post is a typical planning figure for a standard garden fence post.

Worked example

Default inputs: length 30 m, bay width 1.8 m, 2 rails per bay.

Bays = 30 ÷ 1.8 = 16.67, rounded up to 17.
Posts = 17 + 1 = 18.
Rails = 17 × 2 = 34.
Concrete = 18 × 2 = 36 bags (25 kg).

Each point is the calculator re-run with only “Panel / bay width (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Panel / bay width (m)	Posts
0.45	68
0.9	35
1.35	24
1.8	18
2.25	15
2.7	13
3.6	10

How to read the result

For an L-shaped or multi-sided fence, calculate each side separately, because each side rounds up its own last bay; the corner post is shared, so a connected fence with open ends needs the total bays + 1 posts. Gates need stronger posts and often a separate bay calculation. If you enter the panel width only, remember that posts also take up width: with 1.83 m panels and 100 mm posts the post-to-post spacing is about 1.93 m. Taller fences, exposed sites and soft ground need deeper holes and more concrete.

Common mistakes

Forgetting the extra post at the end of the run.
Treating several sides of a garden as one straight run.
Ignoring post width when spacing posts on site.
Not allowing for a gate, which changes bay widths.

Keep in mind: These are estimates. Post depth and footing size depend on fence height, wind exposure and soil; check local rules on boundary fence height and, for tall or retaining fences, consult a qualified professional.

Frequently asked questions

How deep should fence posts be?

A common rule of thumb is about one quarter to one third of the post's above-ground height, but local conditions and product guidance take priority.

How many posts do I need for a 30 m fence?

With 1.8 m bays, 17 bays and 18 posts for a straight run.

How many rails per bay?

Two rails suit lower fences; taller close-board fences often use three.

Sources: BS 1722 — Fences (series of British Standards)

Checked 8 Nov 2026. Typical coverage rates and standard formulas. Always check the product label and local building rules; buy 5–10% extra for waste.

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