How Many Fence Posts, Pickets and Rails Do I Need?

By the SAA Tool editor, an IT support engineer · Last reviewed 9 October 2026 · 7 min read

Fence maths is mostly counting, with one famous trap: a straight fence always has one more post than it has gaps. After that come corners, gates, pickets or panels, rails and concrete. The examples use a 100 ft and a 30 m fence so you can check every number.

It's general information for planning. Heights, boundaries and post depths follow local rules that differ from place to place, and a tall or exposed fence needs a qualified professional's advice.

Posts: sections plus one

Divide the fence length by the largest post spacing you'll accept and round up to get the sections. Posts = sections + 1.

Imperial: 100 ft with posts at most 8 ft apart. 100 ÷ 8 = 12.5, so 13 sections and 14 posts, each section 100 ÷ 13 = 7.69 ft (7 ft 8.3 in).
Metric: 30 m with posts at most 2.4 m apart. 30 ÷ 2.4 = 12.5, so 13 sections and 14 posts, each section 2.31 m.

The classic mistake is buying 12 or 13 posts because the sum said 12.5. Rounding down to 12 sections also stretches each one to 8.33 ft (2.5 m), longer than 8 ft (2.4 m) rails. Spacing is measured centre to centre.

Corners and gates

A corner splits the fence into straight runs, and each run is rounded up on its own. Work out each run, then count the shared corner post once. An L-shaped fence of 60 + 40 ft at 8 ft spacing has 8 + 5 sections, so 9 + 6 posts, less the shared one: 14. Make it 50 + 50 ft and each run rounds up to 7 sections: 8 + 8 posts, less 1, is 15. In metric, 18 + 12 m gives 14 posts and 15 + 15 m gives 15. So a corner adds zero or one post; the Fence Calculator adds one, the safe side.

For a gate, take its width off the length and add one post, because the gate needs a post on each side. A 100 ft fence with a 4 ft gate at one end leaves 96 ft: 12 sections, 13 posts, plus 1 = 14. In metric, 30 m less a 1.2 m gate leaves 28.8 m: 12 sections and 14 posts. Position matters: put the gate where it leaves 46 + 50 ft (or 13.8 + 15 m) and the runs round up to 6 + 7 sections, needing 15 posts. Make both sides whole multiples of the spacing (48 + 48 ft, or 14.4 + 14.4 m) and you're back to 14.

Pickets: width plus gap

A row of pickets has one gap fewer than pickets, so pickets = (length + gap) ÷ (picket width + gap), rounded up, with any gate widths taken off first. Use the real width: the Southern Pine Inspection Bureau explains that a 1×6 board is actually ¾ × 5½ in and a 4×4 post 3½ × 3½ in. Wherever you buy, measure a board.

Imperial: 100 ft is 1,200 in. With 5.5 in pickets touching, 1,200 ÷ 5.5 = 218.2, so 219 pickets. With 0.5 in gaps, (1,200 + 0.5) ÷ 6 = 200.1, so 201.
Metric: 30 m is 3,000 cm. With 14 cm boards touching, 3,000 ÷ 14 = 214.3, so 215. With 1 cm gaps, 3,001 ÷ 15 = 200.1, so 201.

Panels: panel plus post

With ready-made panels, each section is one panel plus one post. Panels = (length less one post width) ÷ (panel width + post width), rounded up, and posts = panels + 1.

Imperial: 100 ft (1,200 in) of 6 ft panels with 3.5 in posts: 1,196.5 ÷ 75.5 = 15.8, so 16 panels and 17 posts. Fifteen panels and 16 posts cover 1,136 in, leaving room for the last post and a panel cut to 60.5 in.
Metric: 30 m of 1.83 m panels with 10 cm posts: 29.9 ÷ 1.93 = 15.5, so 16 panels and 17 posts, the last panel cut to 0.85 m.

Ignoring the posts (100 ÷ 6, or 30 ÷ 1.83) gives 17 panels, one too many.

Rails and spare boards

Rails = sections × rails per section. The 13-section fence with 3 rails per section needs 39 rails (26 with 2, 52 with 4), each spanning about 7 ft 8.3 in (2.31 m), so check the lengths you buy. Panels have rails built in.

Boards split, warp or get cut wrong, so buy spares. The calculator adds 5% to pickets, panels and rails by default (None and 10% are options): 219 pickets become 230 (215 become 226 in metric) and 39 rails 41. Posts and concrete are counted exactly.

Post length, hole size and concrete

Post length = height above ground + depth in the ground. Quikrete gives two rules, both with a hole about three times the post's width. Its Set Posts Without Mixing instructions ask for at least a third of the post's height above ground. Its Setting Posts project sheet goes deeper: a third of the post's overall length (half the height above ground), plus 6 in (150 mm) of gravel underneath for drainage.

For a 6 ft (1.8 m) fence that's 2 to 3 ft (60 to 90 cm) in the ground and posts 8 to 9 ft (2.4 to 2.7 m) long, in holes about 10.5 in wide for a 4×4 or 30 cm for a 10 cm post. Where the ground freezes, the same sheet says structural posts such as deck posts must go at least 6 in (150 mm) below the frost line; your local building office knows that depth.

Concrete per post = (π × hole diameter² ÷ 4, less post width²) × depth.

Imperial: a 12 in hole, 24 in deep, around a 3.5 in post: (113.1 less 12.25) × 24 = 2,420 cu in, or 1.40 cu ft. At 0.60 cu ft per 80 lb bag (Quikrete's yield), 14 posts need 19.6 cu ft, or 33 bags. At 36 in deep: 2.10 cu ft a post, 50 bags.
Metric: a 30 cm hole, 60 cm deep, around a 10 cm post: (0.0707 less 0.01) × 0.6 = 0.0364 m³ (36.4 litres). 14 posts need 0.51 m³, or 44 bags of 25 kg at about 11.7 litres each. At 90 cm deep: 0.76 m³, 66 bags.

With gravel in the bottom, count only the depth the concrete fills. Round the bag total once and use the yield printed on your bags; our guide to how much concrete you need explains yields.

Working it out with the Fence Calculator

Open the Fence Calculator, choose Units and a Fence type: Pickets (boards on rails) or Ready-made panels.
Enter the Fence length, Fence height and Post spacing, at most (or Panel width), then the Corners and Gates. A Width of each gate box appears when you add a gate.
Check the Picket width, Gap between pickets, Rails per section, Post width and the extra.
Leave Work out the concrete for the posts ticked and set the Hole depth, Hole diameter and Concrete bags.
Read the results; Details shows the sections, the post count, post length and concrete per post. Add prices to estimate the cost is optional.

The defaults match the examples above: 14 posts, 230 or 226 pickets, 41 rails, and 33 bags of 80 lb or 44 of 25 kg (after switching units, press Reset to load that system's defaults). It assumes straight runs with even spacing, so on slopes, steps or curves, mark the posts out before you buy. The Length Converter handles mixed units (100 ft is 30.48 m), and the Concrete Slab Calculator covers a concrete strip or pad along the fence.

Buying checklist

Posts: sections + 1 per run, corners counted once, one more per gate, plus a spare.
Pickets or panels: measured widths, gates taken off, 5 to 10% extra or one panel to cut.
Rails: sections × rails per section, each long enough for a section.
Concrete: hole less post for each post, bags rounded up once, gravel for the bottoms.
Not in the calculator: nails or screws, gate hinges and latch, post caps.
Height limits: in England, the Planning Portal says you generally need permission for a fence over 1 m next to a road used by vehicles (or its footpath), or over 2 m elsewhere; Wales may differ. In the US and Canada, check city or county zoning and any homeowners' association rules.
Boundary: confirm the line from your deeds or a survey, and talk to your neighbour.
Buried services: in the US, contact 811 a few business days before you dig; it's free and lists fences among the jobs that need it. In Great Britain, LinesearchbeforeUdig offers a free online search covering its member networks.

Sources

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