Pergola timber calculator
A pergola roof is a grid of main beams and rafters. How much timber it takes depends on the area and how dense you want the shade.
Your measurements
Result
- Pergola area
- 12.00 m²
- Timber for beams
- 24.0 m
Posts, metal anchors and fixings are not included.
How the calculation works
- Pergola area = length × width.
- Timber for beams (m) = area × about 2 m per m² for a simple frame, or about 3.5 m per m² for a dense frame that gives more shade.
Worked example
A 4 × 3 m pergola has 12 m². A simple frame needs about 12 × 2 = 24 m of beams; a dense frame about 42 m.
What the calculator assumes
- The figures include the main beams and the rafters on top at usual spacings.
- Use dried softwood treated for outdoor use, or hardwood.
When to adjust the result
- Posts, metal anchors, screws and post footings are not included.
- Pergolas with a roof (polycarbonate, tiles, membrane) must also be designed for snow load — beams are then heavier and closer.
Planning a pergola
Size the pergola around how you will use it: a table for six with chairs needs roughly 3.5 × 3.5 m of floor space. Posts are usually spaced 2.5–3.5 m apart; longer spans need deeper beams. Leave at least 2.2–2.4 m of clear headroom under the beams.
Posts must be anchored firmly — in concrete footings below the frost line or on bolted post bases on a solid slab — because a pergola with a roof or climbing plants catches a lot of wind. If you add a solid or fabric roof, the structure has to carry snow and wind loads too, and some places treat a roofed pergola as a building that needs approval.
Frequently asked questions
How much timber for a pergola?
Roughly 2–3.5 running metres of beams per square metre of roof, plus the posts.
Which wood is best for a pergola?
Pressure-treated softwood is the usual choice; larch, Douglas fir or hardwoods last longer outdoors.
What timber size for pergola beams?
It depends on the span and on any roof covering. As a rough guide, beams of 45 × 145 mm or larger are used for spans around 3 m; for longer spans or roofed pergolas, ask a supplier or engineer.
Do I need permission to build a pergola?
Often not for a small open pergola, but rules depend on height, size, distance to the boundary and whether it has a roof. Check local planning rules first.