Pergola calculator: posts, beams, rafters and slats
Enter the footprint, how far your beams can span between posts and how far apart the rafters go. This counts the posts, beams, rafters and top slats, spaces them evenly, and turns them into boards to buy.
Outside of posts, along the beams
Outside of posts, along the rafters
What your beam can span — from a span table
On centre
Advanced settings5 1/2" posts 8' tall · 2-ply beams · 12" / 12" overhangs · 1 1/2" slats, 3" gaps
= 5.5"
Footing or bracket to the beam
2 for a beam doubled either side of the posts
Past the end posts, each end
Past the beams, each side
= 1.5"
The gap you would like
A 14 ft × 12 ft pergola takes
Posts
6
3 under each beam
Rafters
13
15 7/8" on centre
Beam boards
4 × 16 ft
Top slats
38
Shaded area
224 sq ft
Laying it out
- Posts3 under each beam, spread evenly — every gap at or under your 8'
- 81 1/4" on centre
- RaftersFlush with both ends of the beams, the spacing evened out so the last bay is not a stub
- 15 7/8" on centre
- Rafter spanBeam centre to beam centre — check this against a span table for your rafter stock
- 138 1/2"
- Slats38 slats across 168" of rafter, one flush at each end — cut a spacer block to the gap
- 3" gaps
Material list
- 6 posts at 96"
- 6 × 8 ft
- 4 beam boards at 192"
- 4 × 16 ft
- 13 rafters at 168"
- 13 × 16 ft
- 38 top slats at 192"
- 38 × 16 ft
Counts and lengths, not sizes. The beam depth for your post spacing, the rafter depth for the span above, and the post and footing are a span table and your building department's call — this page does not guess them.
Where to buy this
- 6 posts, 6 × 8 ftHome DepotLowe's
- 4 beam boards, 4 × 16 ftHome DepotLowe's
- 13 rafters, 13 × 16 ftHome DepotLowe's
- 38 top slats, 38 × 16 ftHome DepotLowe's
This searches the material, not a size we worked out for you — the beam and rafter sizes come from a span table.
Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.
Worked example: a 14' × 12' pergola
With beams allowed to span 8', a 14' × 12' pergola stands on 6 posts — 3 under each beam, 81 1/4" on centre.
Its 4 beam boards are 192" with a 12" overhang at each end, and 13 rafters of 168" sit across them at 15 7/8", spanning 138 1/2" beam to beam.
On top, 38 slats 1 1/2" wide fill the rafters at 3" gaps.
- Posts
- 6 × 96"
- Beam boards
- 4 × 192"
- Rafters
- 13 × 168"
- Top slats
- 38 × 192"
These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.
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About pergolas
- How many posts does a pergola need?
- Enough that no beam spans further than it can between posts — and how far that is depends on the beam's depth, species and grade, so it comes from a span table rather than from this page. Given that maximum, the count is the fewest posts that keep every gap under it, spread evenly: a 14' x 12' pergola whose beams may span 8' gets 3 posts under each beam, 81 1/4" apart on centre, 6 in all.
- How far apart should pergola rafters be?
- 12" to 24" on centre is the usual range, and closer spacing gives more shade and a stiffer top. Whatever maximum you choose, the calculator spreads the rafters evenly from one end of the beam to the other rather than leaving a short bay at the end: at 16" at most on a 16 ft beam, that is 13 rafters at 15 7/8". Check the rafter's own span — beam to beam — against a span table for your stock.
- How do I space the slats on top evenly?
- Pick the gap you would like and let the count follow, the same way baluster spacing works: with a slat flush at each end, the count whose gap lands closest wins. 1 1/2" slats at about 3" across a 168" rafter come out at 38 slats with 3" gaps. Cut a spacer block to the gap rather than measuring each one.
- Why are pergola beams usually doubled?
- A pair of boards bolted either side of the post top is the common way to build a pergola beam without a post cap: the post passes up between them, the bolts carry the load in shear, and two thinner boards are easier to lift and straighter to buy than one heavy timber. It is a construction choice, not a sizing rule — set the boards per beam to one if yours sits on a post cap.
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