What this post calculator does
It works out the number of sections and posts for a fence line, splits the posts into end, corner, gate and line posts, and gives you the actual spacing once the posts are spread evenly. Tick the concrete option to add the volume of concrete and the number of bags for your hole size.
Why post spacing matters
Posts carry everything: the rails, the boards or panels, wind load and the weight of any gate. Wider spacing means fewer posts and holes, but longer rails that can sag and sections that flex in the wind. Rails also come in stock lengths, and 8 ft spacing lets an 8 ft board span one section with little waste.
The spacing you enter is a maximum. Your runs rarely divide exactly, so the calculator rounds up to whole sections and then spreads the posts evenly. That’s why the actual spacing is usually a little less than the number you typed.
How the post count is calculated
sections = ⌈ fenced length ÷ max spacing ⌉ + (runs − 1)
posts = sections + gates + 1 (open run)
posts = sections + gates (closed loop)
- Fenced length is the total length minus gate openings.
- Runs are the straight stretches between corners and gates. Each is rounded up separately, which can add one section per extra run.
- A run of n sections needs n + 1 posts. Neighbouring runs share their corner post; the two runs either side of a gate don’t share, because the opening sits between them.
- A closed loop finishes where it started, so its first and last posts are the same post.
How the concrete is calculated
concrete per hole = π × (hole diameter ÷ 2)² × depth − post area × depth
bags = ⌈ concrete per hole × posts ÷ bag yield ⌉
The post is assumed to reach the bottom of the hole. Bag yields are the typical figures for pre-mixed concrete (an 80 lb bag makes about 0.60 ft³; a 25 kg bag about 11.7 litres). The exact yield is printed on the bag.
Worked example
The calculator opens with a 150 ft fence, posts at most 8 ft apart, two corners and one 4 ft gate:
- Fenced length: 150 − 4 = 146 ft.
- Two corners and one gate make four runs.
- Sections: 146 ÷ 8 = 18.25 → 19, plus 3 for the extra runs = 22.
- Posts: 22 + 1 gate + 1 = 24 posts (as few as 21 if every run divides evenly).
- Concrete: 10 in × 24 in holes around 3½ in posts hold 0.92 ft³ each; 24 × 0.92 = 22.1 ft³ ÷ 0.375 ft³ = 58.9 → 59 × 50 lb bags.