Pergola Calculator
A planning materials list for a freestanding pergola: posts, doubled beams, rafters and purlins counted from the footprint with standard sizing rules of thumb.
A pergola counted by the rules of thumb
A pergola is a grid of shade, and its lumber follows a few steady rules. Posts stand at the corners, with a mid post added on any side longer than 12 ft, so no beam span runs away. The two main beams cross the post tops along the length — doubled, one board each side of the posts, in the classic build — and every member is allowed an extra foot of length at each end for the decorative overhang that makes a pergola read as a pergola. Rafters cross the beams at 16 in centers across the width, and purlins — the light top slats that throw the striped shade — run back along the length about a foot apart.
Posts = corners + mids on sides over 12 ft · Rafters = length ÷ 16 in + 1 · Beam & rafter length = span + 2 ft of overhang
Example: a 12 × 10 ft pergola, 8 ft high, with doubled beams: 4 posts (6 × 6, 8 ft), 4 beam boards of 2 × 8 at 14 ft, 10 rafters of 2 × 6 at 12 ft (11 to buy with the allowance), and 13 purlins (15 to buy) at 14 ft.
Read this as what it is: a planning list, built on the standard rules of thumb above. Pergolas are structures — spans, post footings, attachments and wind loads are governed by local code and soil, and lumber sizes here (2 × 8 beams, 2 × 6 rafters, 2 × 2 purlins) assume the modest spans in the example's class. Confirm the design locally before building, especially for large footprints, attached pergolas, or anywhere snow or high wind is a factor. Post lengths are given at the structure height: if the posts are buried in concrete rather than mounted on footings, add the bury depth to each post.
Frequently asked questions
How far apart can pergola posts be?
The planning rule used here adds a post whenever a side passes 12 ft, keeping beam spans in the range a doubled 2 × 8 handles comfortably as a shade structure. Longer spans are possible with bigger beams — but that is engineered territory, and the design should be confirmed locally rather than stretched from a rule of thumb.
What size lumber is standard for a pergola?
6 × 6 posts are the common choice (4 × 4 reads spindly past small sizes), beams are typically 2 × 8 or 2 × 10, rafters 2 × 6 or 2 × 8, and purlins 2 × 2 or 1 × 4 laid as the top shade layer. Cedar and pressure-treated pine are the usual species; the calculator counts pieces, and the sizes named are the typical set.
How much shade does a pergola actually give?
The striped shade of the classic open grid cools a patio noticeably without darkening it — roughly half the direct sun at midday with purlins a foot apart, more as the sun angles. Closer purlin spacing or a shade cloth layer increases it; the calculator's purlin count at 12 in spacing is the classic look and a fair middle ground.
Does a pergola need footings?
Yes — each post stands on a footing: typically a concrete pier or a buried post in concrete, sized by local frost depth and code. Freestanding surface-mount posts on post bases bolted to a slab are the other accepted pattern. Either way the footing work is its own small concrete job — the post hole and concrete calculators will count it.
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