A 100 ft straight run at 8 ft spacing is 14 posts, not 12.
That is the number that sends people back to the yard before the first hole is poured. You divide 100 by 8, get 12.5, round it down, and load twelve posts. The run still needs a post at 0 and a post at 100. Twelve cannot cover both ends and the line in between.
The open-run formula is:
posts = ceil(length ÷ spacing) + 1
Ceil means you never stretch the last bay past the spacing you picked. The leftover 4 ft on a 100 ft run is a short bay, not a missing post. The +1 is the far-end post. Drop it and every run is short by one.
This chart is for a straight, open run — a post at both ends, no gate, no corners. Corners, gates, and a closed yard change the count. Estimate only — not engineering or building-code advice.
| Run | 6 ft | 8 ft | 10 ft |
|---|---|---|---|
| 50 ft | 10 | 8 | 6 |
| 100 ft | 18 | 14 | 11 |
| 150 ft | 26 | 20 | 16 |
| 200 ft | 35 | 26 | 21 |
How many posts for 100 feet of fence?
For 100 feet of fence at 8 ft centres, order 14 posts.
Walk the line: a post at 0, then 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, and the end post at 100. That last gap is 4 ft. Short is fine. Skipping the end post because 100 is not a clean multiple of 8 is how a job stalls at 4 pm.
At 6 ft spacing, 100 ft is 18 posts. At 10 ft spacing, 100 ft is 11 posts. Those counts are one straight, open run. A backyard that turns two corners is not one run. Split the tape at every corner, count each segment, then subtract shared corners you counted twice.
The formula, on a tape
Measure along the ground. On a slope, posts follow grade, not the flat distance on a plot plan.
Pick a maximum centre-to-centre spacing. Divide length by spacing. Round up — that is the number of bays. Add one. That is the number of posts.
50 ft at 8 ft: 6.25 → 7 bays → 8 posts. 150 ft: 18.75 → 19 bays → 20 posts. 200 ft: 25 even → 25 bays → 26 posts.
A clean multiple is the trap. 80 ft at 8 ft is 10 bays and 11 posts. The math looks “done” at 10, so people stop one post short. Do not round the count down. The long last bay is where rails sag.
Three mistakes that short the order
Dropping the +1. Length divided by spacing is the number of spaces, not posts. n gaps need n + 1 posts. Forget the +1 and every run walks off the truck light.
Counting a gate as one post. A gate is an opening with a post on each side — hinge and latch. A single leaf needs two gate posts. A double driveway gate still uses two main posts in most layouts. Do not punch a hole in the line-post formula and call that one post.
Treating corners as free. A corner post is a real post, usually heavier, and it is shared by two runs. Run the formula on the south side and the east side, add the totals, and you counted that corner twice. Sketch the dots. Count the dots. A four-sided yard with no gates is four corners plus line posts on each side — not four independent fences.
How far apart should fence posts be?
Most residential wood privacy and picket in the US and Canada sits on 6 ft or 8 ft centres. Eight feet is the common yard spacing. Six feet is stiffer, uses more posts, and is the better call in wind, on a slope, or with heavy panels.
Ten feet shows up on farm mesh and some pipe-rail — not a default for a 6 ft privacy wall. Panel systems in the UK, Australia, and New Zealand follow the panel module, often 1.8 m or 2.4 m. Vinyl and metal kits follow the manufacturer’s panel width. Do not pick a spacing the rails will not span.
Closer is stronger. Wider bays need heavier rails. Spacing is centre to centre, not the clear gap between faces.
Do I need extra posts for a gate?
Yes. Plan two posts for every gate, then treat the remaining fence as separate short runs.
Subtract the opening from the run. Split what is left on either side of the gate. Each wing gets the open-run formula of its own: a corner or end on one side, a gate post on the other.
Example: 100 ft of frontage, 8 ft spacing, one 4 ft walk gate in the middle. You do not have a 100 ft run. You have two wings of 48 ft. Run the formula on each wing. The two gate posts are the inner terminals. They replace line posts at that spot, and they still belong on the list as gate posts — usually a larger section.
Gate posts are often 4×4, 6×6, or steel, and they take more concrete than a line post. For post-hole bags, use Concrete Mix Master.
If the fence returns to itself, the last post of the last run is the first post of the first run. Add the sides, then subtract one for every shared corner. A simple rectangle is four shared corners. Then add any gate posts that are not already on a corner.
Mark the line with string. Spot every hole before you dig. Move a mark off a drain, a tree root, or a buried line. Buy a spare line post. A split is cheaper than a second trip.
This count is an estimate only — not engineering or building-code advice. Product span tables, wind, frost depth, and local rules can require closer spacing or larger posts than a takeoff formula. When the layout has more than one run, Fence Takeoff Master turns lengths, spacing, and gates into a post count plus rails and gravel. The rest of the EBN Build Calculators cover concrete bags, cut lists, and the other jobsite math.
Open Fence Takeoff MasterEstimate only — not engineering or building-code advice.