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Fence posts calculator
Put your fence length, post spacing, bags per post and price per post in: a 100 ft fence at 8 ft spacing needs 13 intervals and 14 posts, 14 bags at 1 bag a post, $168 at $12 a post.
★ CONSTRUCTION · FENCES · POSTS
Fence Posts Calculator: 14 Posts for a 100 ft Fence
On the defaults — a 100 ft fence, 8 ft spacing, 1 bag a post, $12 a post — you need 14 posts, 14 bags, $168 total. This is the fence posts tool: divide the run by the spacing, add the end post, and order the posts once — a fence one post short waits for no second delivery.
What does fence posts needed 14 posts mean?
It means a 100 ft fence with posts 8 ft apart needs 14 posts: 13 intervals between posts, plus one post at the end of the run. The intervals are the length divided by the spacing, rounded up, because a fence cannot end part-way through an interval — 100 ÷ 8 is 12.5, so the run needs 13 intervals, and the last interval is simply shorter than 8 ft. The bags follow the posts, 14 posts at 1 bag each is 14 bags, and the cost follows the posts too, 14 posts at $12 each is $168. The post count is the shopping number, while the interval count is the setting-out number on the ground: mark 13 intervals and the 14th post closes the run.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Intervals | length ÷ spacing, rounded up | 13 intervals — 100 ÷ 8, rounded up. |
| Posts | intervals + 1 end post | 14 posts — 13 + 1. |
| Bags | posts × bags per post | 14 bags — 14 × 1. |
| Total cost | posts × price per post | $168 — 14 × $12. |
Three things to check before you trust the number
First, measure the run on the ground, not off a plan: a run that slopes or steps needs its slope length, and a gate breaks the run into two shorter runs, each with its own end posts. Second, keep the spacing honest: 8 ft suits most timber fences, but a tall fence or a windy site wants tighter spacing, and every foot tighter adds posts fast. Third, remember the last interval is short — 100 ft at 8 ft spacing leaves a 4 ft final interval, so the end post lands at 100 ft, not at 104 ft. For the price side of the same fence, the fence cost calculator counts rails, boards and labour too.
Worked example
100 ft fence, 8 ft spacing, 1 bag a post, $12 a post. Intervals: 100 ÷ 8 = 12.5, rounded up to 13. Posts: 13 + 1 = 14. Bags: 14 × 1 = 14. Cost: 14 × $12 = $168. In practice that means buying 14 posts and 14 bags, setting the first post at 0 ft and the last at 100 ft, with the final interval measuring 4 ft — and keeping one spare bag, since a post in soft ground drinks more concrete than the plan allows.
Every branch, checked
With a 120 ft fence instead of 100 ft, the intervals rise to 15 and the posts to 16, costing $192. With 6 ft spacing instead of 8 ft, the intervals rise to 17 and the posts to 18, costing $216, since tighter spacing adds posts across the whole run. With 2 bags a post, the posts stay 14 and the cost stays $168, but the bags double to 28. With $15 a post, the posts and bags stay 14 but the cost rises to $210, since price moves only the money. Length, spacing, bags and price each move the fence on their own, so a wrong spacing figure is as costly as a wrong length.
The reverses, worked backwards
The length behind 14 posts is 100 ft: at 80 ft the need is 11 posts (80 ft fence, 8 ft spacing, 10 intervals, 11 posts, 1 bag a post, 11 bags, $12 a post, $132 total); at 160 ft it is 21 posts (160 ft fence, 8 ft spacing, 20 intervals, 21 posts, 1 bag a post, 21 bags, $12 a post, $252 total). The spacing behind 14 posts is 8 ft: at 6 ft the need is 18 posts (100 ft fence, 6 ft spacing, 17 intervals, 18 posts, 1 bag a post, 18 bags, $12 a post, $216 total); at 10 ft it is 11 posts (100 ft fence, 10 ft spacing, 10 intervals, 11 posts, 1 bag a post, 11 bags, $12 a post, $132 total). The price behind $168 is $12: at $10 the need is 14 posts, $140 total (100 ft fence, 8 ft spacing, 13 intervals, 14 posts, 1 bag a post, 14 bags, $10 a post, $140 total); at $15 it is 14 posts, $210 total (100 ft fence, 8 ft spacing, 13 intervals, 14 posts, 1 bag a post, 14 bags, $15 a post, $210 total). Each reverse moves one input and holds the rest, so the step is exactly one length, spacing or price change.
What the defaults assume
The defaults assume a single straight 100 ft run, 8 ft spacing, 1 bag of concrete per post, and $12 a post for the post alone. They do not assume gates, corners, slopes, rails, boards or labour — a corner post and a gate post each add a post to their run, and a sloped run needs its slope length, not its flat length.
Who uses this
- A homeowner pricing a garden fence: the 14-post count settles the timber order before the first hole is dug.
- A fencer quoting a job: the intervals, posts and bags off the calculator are the lines on the quote, not a guess.
- A DIY builder setting out a run: the 13-interval setting-out tells where each hole goes, including the short final interval.
- A buyer comparing post prices: the same maths at $10 and $15 a post shows what the post choice is worth in dollars.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the length and the spacing. Real runs bend, step and stop at gates, and the tape on the ground overrides any planning figure; a run measured flat under-counts a sloped run, and a spacing read off a neighbour fence may not suit a taller fence. Treat 14 posts as the planning count for a straight 100 ft run at 8 ft spacing, and walk the line with a tape before ordering — gates and corners each add posts the single-run maths never sees.
Cite this calculator
Bright Side Kit, “Fence Posts Calculator: 14 Posts for a 100 ft Fence”, https://brightoolbox.com/fence-posts-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (100 ft fence, 8 ft spacing, 1 bag a post, $12 a post) so the 14-post result can be checked.
Frequently asked questions
How many fence posts do I need for 100 ft?
14 posts at 8 ft spacing: 100 ÷ 8 = 12.5, rounded up to 13 intervals, plus 1 end post = 14 posts, with 14 bags at 1 bag a post and $168 at $12 a post.
How do I work out fence posts from a fence length?
Divide the length by the spacing, round up to whole intervals, then add one post. A 100 ft fence at 8 ft spacing is 13 intervals and 14 posts; a 120 ft fence at the same spacing is 15 intervals and 16 posts.
Why do I add one post after dividing?
Because intervals count the gaps between posts, and a run needs a post at both ends: 13 gaps need 14 posts. Forgetting the end post is the classic fence shortage — one post short, with the last rail hanging.
What if my fence length is not a multiple of the spacing?
The last interval is simply shorter: 100 ft at 8 ft spacing leaves a 4 ft final interval, and the end post still lands at 100 ft. Never stretch the run to 104 ft to make the intervals even.
How many bags of concrete per fence post?
One bag a post on the defaults, but ground conditions decide: soft or sandy ground takes more, firm clay takes less. The concrete bag calculator sizes the concrete by hole size if you want it measured, not assumed.
Does a gate change the post count?
Yes — a gate breaks one run into two runs, each with its own end posts, and gate posts are usually heavier. Count each side of the gate as its own run on this calculator, then add the gate posts.