How to use this calculator
- Choose panel fence or post-and-rail / picket.
- Enter the fence length and the panel width or post spacing.
- Add extra posts for corners and gates.
- Enter the post hole size to see the concrete needed.
The formula
Spacing = panel width + post width (panel fence) or post spacing
Sections = length ÷ spacing, rounded up · Posts = sections + 1 + extra posts
Pickets = length ÷ (picket width + gap), rounded up
Concrete per hole = π × (hole Ø ÷ 2)² × depth − post area × depth
A straight run always has one more post than sections. The last panel or section may need cutting down, which is why the count rounds up.
Post concrete is the hole volume minus the part the post fills.
Worked example
A 20 m fence with 1.83 m panels and 100 mm posts needs 11 panels and 12 posts.
Holes 300 mm wide and 600 mm deep need 0.481 m³ of concrete — 42 bags of 25 kg.
Tips before you buy
- Set the first and last posts, run a string line between them, then set the rest.
- Slope the concrete away from the post at the top so rain runs off.
- Check boundary lines and any height limits before you build.
Frequently asked questions
›How many fence posts do I need?
For a straight run: the number of sections plus one. Add one more for each corner and each gate side.
›How deep should fence posts be?
A common rule is about a third of the post's height above ground, and deeper where the ground freezes. Local advice comes first.
Sources
- Method: sections + 1 posts; post concrete = hole volume − post volume. Bag yields as in the concrete calculator.
Formulas last reviewed: September 29, 2026. How we check our formulas