How it works
Posts stand on lines under each beam. The outer posts sit where the beam overhang ends; more posts are added evenly until no gap between posts is larger than your maximum.
Beam rows run across the depth. A freestanding pergola has a front and a back beam; an attached one has a ledger on the wall instead of the back beam. When the rafter span would be too long, extra beam rows with posts are added evenly.
Rafters are spaced along the width with the same rules as the equal spacing calculator, and each rafter is one piece over the full depth.
Beams and top slats longer than your stock are joined only over a post or a rafter. Every member type gets its own optimised cutting plan from the stock lengths you can buy.
What the calculator assumes
- This is a planning and material tool, not a structural design. Member sizes, spans, post spacing, connections, wind and snow loads and foundations must follow the supplier's system or a structural engineer, and local building rules.
- All members are bought in the same stock lengths you enter; each member type is cut separately.
- Rectangular pergola with a flat (or slightly sloped) roof plane; no roofing, gutters or bracing included.
Questions
- How far apart can the posts be?
- That depends on the beam size, timber grade and loads, so the calculator asks you for the maximum. Use the value from your kit instructions, timber span tables valid where you live, or an engineer.
- What does “boards per beam” mean?
- Many wooden pergolas use a double beam — two boards bolted to both sides of the posts. Choose 2 and the cut list doubles the beam boards.
Planning aid only. It does not size structural members or check loads — follow the manufacturer's instructions and your local building rules.