Fence Calculator (posts, rails, pickets, concrete)
Work out the posts, rails, pickets, and concrete a fence run needs, with the real post spacing after rounding.
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Posts needed by run length
Post counts and the real spacing they work out to, for the three usual nominal spacings. The actual gap is always at or below the nominal, because the sections have to divide the run exactly.
| Run | 6 ft nominal | 8 ft nominal | 10 ft nominal |
|---|---|---|---|
| 20 ft | 5 posts @ 5 ft | 4 posts @ 6.67 ft | 3 posts @ 10 ft |
| 40 ft | 8 posts @ 5.71 ft | 6 posts @ 8 ft | 5 posts @ 10 ft |
| 50 ft | 10 posts @ 5.56 ft | 8 posts @ 7.14 ft | 6 posts @ 10 ft |
| 60 ft | 11 posts @ 6 ft | 9 posts @ 7.5 ft | 7 posts @ 10 ft |
| 80 ft | 15 posts @ 5.71 ft | 11 posts @ 8 ft | 9 posts @ 10 ft |
| 100 ft | 18 posts @ 5.88 ft | 14 posts @ 7.69 ft | 11 posts @ 10 ft |
| 150 ft | 26 posts @ 6 ft | 20 posts @ 7.89 ft | 16 posts @ 10 ft |
| 200 ft | 35 posts @ 5.88 ft | 26 posts @ 8 ft | 21 posts @ 10 ft |
Working out fence materials
A fence is a run of identical sections, so most of the quantities follow directly from the length. The one that trips people up is the posts: a straight run needs one more post than it has sections, because both ends need one.
Sections = ⌈run / spacing⌉ · Posts = sections + 1 · Pickets = run / (width + gap)
Worked example
A 100 ft fence with posts no more than 8 ft apart: 100 / 8 = 12.5, which rounds up to 13 sections and therefore 14 posts. The real spacing becomes 100 / 13 = 7.69 ft — slightly tighter than the nominal, which is what you want. With two rails per section that is 26 rails, and 6 inch boards butted together need 100 × 12 / 5.5 ≈ 219 pickets.
Why the spacing comes out tighter
The figure you enter is a maximum, not a target. Rounding the section count up and then dividing the run evenly always gives a spacing at or below what you asked for, which keeps every bay identical and the rails within their span rating. The alternative — full-width bays plus a narrow one at the end — is quicker to calculate and looks noticeably worse.
Post holes and concrete
The rule of thumb is that a third of the post length goes in the ground, and the hole is about three times the post's width across. For a standard 4×4 (actually 3.5 in) that means a 10–12 inch hole, 2 ft deep for a typical 6 ft fence. The concrete needed is the hole volume minus the buried part of the post, which this calculator works out — and it is why a fatter post needs slightly less concrete, not more.
In climates that freeze, holes need to go below the local frost line or the posts will lift over winter, and that depth can be well beyond 2 ft. Check the requirement locally before digging.
Pickets and gaps
A solid privacy fence has the boards butted with no gap. A picket or shadowbox fence has a deliberate space — set the gap and the count adjusts. Note that nominal lumber is smaller than its name: a "6 inch" fence board is usually 5.5 in, and using 6 will leave you short by roughly 9%.
What this doesn't cover
Gates, corners, and end assemblies need extra posts, and a corner post serves two runs — so for an enclosure, calculate each straight side separately and subtract the shared corner posts. Slopes need either stepped or racked panels, both of which use more material than the flat run. For the concrete on its own, see theconcrete calculator.