The formula
posts = length ÷ spacing + 1, unless the perimeter closes on itself
Where it comes from
The "plus one" is the thing to remember: over twenty metres with posts every two and a half there are eight bays but nine posts, because one goes at each end. The exception is a perimeter that closes on itself, where the last post is the first one and the posts equal the bays exactly. The other useful figure is the actual spacing: if the length is not a multiple of the spacing, it is better to share the remainder across every bay than to leave one short bay at the end, which shows.
How to work it out by hand
- Divide the length by the spacing and round up: those are the bays
- Add one if the fence does not close on itself
- Share the length across the bays for the actual spacing
- Work out the concrete in one hole and multiply by the posts
What is worth knowing
Footing depth is usually set at a third of the height above ground, and never less than 50 cm on a fence 1.8 m high: what pushes a fence over is not weight but wind, levering on the post. The hole is dug wider at the bottom than at the mouth, like an upturned cup, so the block of concrete cannot pull straight out. And on sloping ground the fence is stepped in level bays rather than following the slope: it is the only way to keep the panels square.