A fence is not one project but one small module repeated to the end of the garden: a post in a hole, rails spanning to the next post, boards fixed to the rails. Because the module repeats, every quantity on the materials list — posts, bags of concrete, rails, boards — comes from simple division once you have fixed two decisions: the post spacing and the board pattern.
This guide works those decisions through: spacing by fence type, how deep posts go and how wide their holes are, concrete versus gravel backfill, bracing while the concrete cures, and the board arithmetic that fills the run. A single worked example — 20 m of board fence — carries the whole calculation.
Posts = run ÷ spacing, rounded up, plus one. A 20 m run at a maximum 2.4 m spacing is 20 ÷ 2.4 = 8.33 → 9 bays, 10 posts, set evenly at about 2.22 m. Boards per bay = bay length ÷ (board width + gap): at 150 mm boards with 10 mm gaps, each 2.4 m bay takes exactly 15 boards. Allow 5–10 percent waste on boards, and bury posts a third to half of their above-ground length — about 0.6–0.9 m for a 1.8 m fence. The Fence Calculator turns these rules into a materials list.
Post Spacing by Fence Type
Spacing is set by what the fence is made of. Rigid panels demand exact spacing; boards on rails tolerate a range.
| Fence type | Typical post spacing | What sets it |
|---|---|---|
| Board fences (closeboard, palisade, board-on-board) | 1.8–2.4 m (6–8 ft) | Rail span: 2.4 m is the usual maximum for two or three rails without sag |
| Panel fencing | Exactly the panel width — commonly 1.83 m (6 ft) | The panel is a rigid module; posts must land on its edges |
| Post-and-rail / paddock | Often wider, up to about 3.6 m | Few, wide bays are traditional for stock fencing on level ground |
| Retaining or steep slopes | Shorter bays | Stepped panels and racked boards both eat length quickly |
Shorter is stronger, always. Every extra metre of bay is more rail to sag under its own weight and more wind area carried by each post. Where a fence faces prevailing wind or carries a heavy board build, dropping from 2.4 m to 1.8 m spacing costs a third more posts and buys a visibly stiffer fence. Spacing figures here are typical practice; regional conventions and manufacturer instructions vary, so treat them as starting points.
The Worked Example: 20 m of Board Fence
These are the defaults in the Fence Calculator: a 20 m run, a maximum post spacing of 2.4 m, boards 150 mm (6 in) wide fixed with 10 mm gaps. Working it by hand:
- Bays: 20 ÷ 2.4 = 8.33 — you cannot build a third of a bay, so round up to 9 bays.
- Posts: 9 bays + 1 = 10 posts.
- Spacing: divide the run evenly: 20 ÷ 9 ≈ 2.22 m per bay. An even rhythm reads better than eight long bays plus one stub. (The alternative — strict 2.4 m bays — gives 8 × 2.4 = 19.2 m plus a 0.8 m closing bay; the post count is 10 either way.)
- Rails: a fence this tall typically takes 3 rails per bay: 9 × 3 = 27 rails cut to 2.22 m. From standard 4.8 m rail stock, two rails per length with an offcut: 27 ÷ 2 = 13.5, so 14 rail lengths.
- Boards: each board occupies 150 mm + 10 mm = 160 mm of run. Per bay: 2.22 ÷ 0.16 = 13.875 → 14 boards (the last one trimmed). Total: 9 × 14 = 126 boards.
- Waste: add 5–10 percent for cuts, splits and rejects: 126 × 1.05 ≈ 133 to 126 × 1.10 ≈ 139 — order about 135 boards, or round to whole packs.
As a check on the board arithmetic, a full 2.4 m bay at the same pattern takes 2.4 ÷ 0.16 = 15 boards exactly — the numbers only work that cleanly at exactly 2.4 m, which is one reason it is the traditional maximum spacing.
How Deep: Post Embedment
The rule of thumb that travels: bury about one third to one half of the post's above-ground length. A 1.8 m (6 ft) fence therefore wants 0.6–0.9 m of post in the ground, which means buying 2.4–2.7 m posts — the classic "8 ft post for a 6 ft fence". Shorter fences can lean toward the one-third end; tall fences, exposed sites and gates push toward half.
In climates where the ground freezes, there is a second constraint: the hole should reach below the local frost line, the depth to which soil freezes in winter. Frost heave — frozen soil expanding and gripping the post — will walk a shallow post upward an inch or two every winter until the fence wobbles. Frost depth varies enormously by region, from negligible in warm climates to well over a metre in continental ones, so ask locally. Practice genuinely varies by country and soil; the third-to-half rule and the frost-line rule together cover most of it.
Hole Size and Concrete Volume
The hole wants to be roughly three times the post's width across: a 100 mm (4 in) square post gets a 250–300 mm (10–12 in) hole. Depth follows the embedment rule — 600 mm (24 in) is typical for a 1.8 m fence, though it varies with the frost line above.
Each hole is a small concrete pour, and the volume is worth knowing before you buy bags. A 250 mm hole, 600 mm deep, holds π × 0.1252 × 0.6 ≈ 0.03 m3 — about 30 L, call it 70 kg of mixed concrete. The worked example's 10 holes need about 0.3 m3 in total. Run that volume through the Concrete Slab & Footing Calculator (a post hole is just a small, deep footing) and it converts to bag counts; the fuller treatment of estimating concrete — waste factors and when to order ready-mix — is in How much concrete do I need?
Concrete or Compacted Gravel?
Both backfills are legitimate, and installers argue about them in every country. The honest trade-offs:
- Concrete anchors the post rigidly in one pour. It is the usual choice for gate posts, tall fences, soft or sandy soil, and anywhere wind loading is serious. Its weakness is water: a flat concrete collar can trap moisture against the post at ground level. The fix is simple — dome the concrete a little above grade so rain sheds off, and slope the surrounding soil away.
- Compacted gravel sets the post in a hole backfilled with angular gravel, rammed in layers (75–100 mm at a time) with a post rammer or the butt of a bar. It drains freely, so timber posts stay drier at the ground line — and posts are easier to plumb-adjust, replace or remove later. It suits dry climates, lighter fences and well-draining soils; in very soft ground it can work loose where concrete would not.
For timber posts, whichever backfill you choose, use timber rated for ground contact and expect the ground line — that few centimetres where post meets soil and oxygen — to be where fences eventually fail. Creosote-style banding at the base and a gravel collar both push that day further out.
Bracing While the Concrete Cures
Concrete does not wait politely for you to fix mistakes. Before any pour: set each post, check it plumb in both directions with a level, check it in line with a string stretched between the end posts, then brace it with temporary diagonal battens nailed to stakes. Brace the run, pour the run.
Concrete is typically firm enough to take light loads after 24–48 hours, but it reaches full strength over weeks; the details are in our concrete curing guide. Fix rails and boards only when the posts will not move under them. Gravel-set posts can be loaded almost immediately — compact carefully instead — but re-check plumb after the first heavy rain.
Rails per Bay and Board Counts
Rail count follows fence height: two rails for fences up to about 1.2 m (4 ft), three rails for taller board fences — typical practice, with the top rail a little below the post tops and the bottom rail close enough to the ground that board ends are not flapping in the wind. The worked example's 3 rails × 9 bays = 27 rails, or 14 lengths of 4.8 m stock, is the arithmetic for any run.
The board formula is worth having as a line: boards = bay length ÷ (board width + gap), per bay, summed, plus 5–10 percent waste. The gap is a design decision with real leverage — at the worked example's 150 mm board with a 10 mm gap the pitch is 160 mm; widen the gap to 25 mm and the pitch becomes 175 mm, dropping each 2.22 m bay from 14 boards to 2.22 ÷ 0.175 ≈ 13 boards, and the run from 126 to about 117. A few extra millimetres of air between boards is the cheapest board saving there is.
Gate Posts: The Exception
Everything about a gate post is heavier than the run. A swinging gate loads its posts dynamically, in both directions, every time it opens — so gate posts are a bigger section (125 × 125 mm or 150 × 150 mm in place of 100 × 100 mm is typical), set deeper, and concreted regardless of what the rest of the run sits in. The gate's width and its hardware's clearances decide the spacing between the pair, so fix the hardware first and set the posts to the hardware maker's instructions rather than to the fence's rhythm.
Plan It With the Fence Calculator
The Fence Calculator takes the run length, post spacing, board width, gap and rail count, and returns posts, bays, rails and boards — waste included — the same way the worked example above does. Its defaults are that exact 20 m run.
Open the Fence CalculatorFrequently Asked Questions
How far apart should fence posts be?
Board fences typically run 1.8–2.4 m between posts, with 2.4 m the usual rail-span maximum. Panel fences space posts exactly to the panel width — commonly 1.83 m (6 ft). Shorter spacing is always stiffer; each extra metre of bay adds rail sag and wind load per post.
How deep should a fence post go?
Bury one third to one half of the above-ground length: a 1.8 m fence wants 0.6–0.9 m in the ground, so 2.4–2.7 m posts. In freezing climates the hole should also reach below the local frost line, whose depth varies by region — frost heave lifts shallow posts over winters. Gates, tall fences and exposed sites all want more.
How many posts do I need for a 20 m run?
Run ÷ spacing, rounded up, plus one. At a maximum 2.4 m: 20 ÷ 2.4 = 8.33 → 9 bays and 10 posts — either evenly spaced at ≈2.22 m, or eight 2.4 m bays plus a short 0.8 m closing bay. Ten posts either way.
Is concrete or gravel better for setting fence posts?
Both are legitimate. Concrete anchors rigidly — the usual choice for gates, tall fences and soft ground; dome it above grade so water sheds off the post. Compacted gravel, rammed in 75–100 mm layers, drains freely and makes posts easier to adjust or replace. Regional practice varies; timber rated for ground contact matters either way.
How do I calculate how many boards I need?
Boards per bay = bay length ÷ (board width + gap). At 150 mm boards with 10 mm gaps the pitch is 160 mm: a 2.4 m bay takes 15 boards exactly, a 2.22 m bay about 14. Sum the bays and add 5–10 percent waste — 9 bays × 14 = 126 boards, so order around 135.
Related MyHouseLogic Guides
Posts set in concrete are a concrete pour like any other; the rest of the garden's numbers follow the fence.
- How much concrete do I need? — the fuller estimating method: waste factors, bag counts, and when a pour justifies ready-mix.
- Concrete curing guide — what those 24–48 hours and following weeks actually do, and what a post hole can safely take when.
- Concrete Slab & Footing Calculator — run the post-hole volumes (or the shed slab the fence surrounds).
- All Garden & Outdoor resources — fences, decks, beds and the calculators that size them.
The bottom line: divide the run by the spacing, round up, add one post; bury a third to half of the post's above-ground length in a hole three times its width; count boards from board-plus-gap, with 5–10 percent waste. Concrete anchors, gravel drains — both work, domed or compacted properly. And set gate posts heavier than you think they need to be.