Pergola Post Spacing Calculator

What this calculation does not cover

• Every answer describes a freestanding four-sided frame: the total adds the posts on two long sides and two short sides and then subtracts the four shared corners, and there is no input for a pergola attached to a house or garden wall, where one whole run of posts is replaced by a ledger and the count drops.
• Only two maximum spans exist in the dropdown, 6 ft and 8 ft, and whichever you choose is divided into both dimensions at once, so a heavier beam that clears more than 8 ft between posts, or a layout that runs a long beam one way and lighter members the other, cannot be described here.
• Each side rounds its bays up on its own, so the two sets of centres above are usually different from each other and both sit under the maximum you picked — a 4 m side at the 8 ft setting is two bays of roughly 6.6 ft. That is the frame set out square. What it is not is a beam layout: the centres are post positions, and a beam carrying them still has to be checked against its own span.
• The metric option labels are rounded while the arithmetic is not — the option shown as 1.8 m divides by 6 ft (1.829 m) and the one shown as 2.4 m divides by 8 ft (2.438 m) — so on some footprints the bays behind the answer run a few centimetres wider than the metric figure you thought you had selected.
• The length and width you type are treated as the post run itself, so no allowance is made for the beam and rafter tails that normally cantilever past the corner posts, nothing distinguishes an overall outside dimension from centre-to-centre post positions, and only true rectangles are handled — an L-shaped or angled footprint has to be split and each part run separately.
• What comes back is a count of posts and nothing more: no post height or section size, no footing depth or embedment, no diagonal or knee bracing against racking, and no post bases, brackets or fixings.