A 24 metre fence at 2.4 metre post spacing needs 11 posts, not 10. Dividing 24 by 2.4 gives 10, and 10 is the number of bays, not the number of posts. Each bay needs a post at both ends, and adjacent bays share one, so the count is always bays plus one.
It is the most reliably repeated error in construction estimating, and it is always discovered at the far end of the run with everything else already installed.
The same arithmetic, four different trades
The pattern turns up wherever evenly spaced things span a length:
- A 24 metre fence at 2.4 metre spacing: 11 posts, from the fence calculator.
- A 6 metre wall at 400 millimetre centres: 16 field studs, from the wall stud calculator.
- A 12 metre building at 24 inch centres: 21 trusses, from the roof truss calculator.
- A 6 metre wall of board and batten at 400 millimetre spacing: 16 battens, from the board and batten calculator.
In every case the division gives the gaps and the plus one gives the closing member. None of these calculators make you remember it, which is rather the point of them.
Balusters run the other way
Railing spindles invert the logic, and it catches people who have learned the first rule. With spindles between two posts, the gaps outnumber the spindles: 21 spindles produce 22 gaps, because there is a space at each end against the posts as well as between every pair.
That is why the baluster spacing calculator divides the leftover length by the spindle count plus one. Getting it the other way round produces end gaps wider than the rest, which looks wrong and, if the gap exceeds the code maximum, is wrong.
Deck footings multiply the error
A grid compounds it in both directions. A 6 by 4 metre deck at 2.4 metre maximum spacing needs 4 footings along each of 3 lines: 12 in total, not the 6 that dividing both dimensions would suggest.
Because the plus one applies to each axis independently, the error is larger for a grid than for a line. The deck footing calculator handles both axes, and the actual spacing usually comes out tighter than the maximum entered, since the count rounds up to whole bays.
Why it survives
Because the division feels complete. Length over spacing produces a clean number that looks like an answer, and nothing about it signals that it is counting the wrong thing.
The habit that prevents it is to sketch three posts and two bays before ordering anything. Three and two is visibly right, and it makes the general pattern obvious in a way that arithmetic on a 24 metre run does not.
For the holes those posts sit in, see the fence post depth calculator. For the related problem of turning an area into an order quantity, see the piece on ordering materials.