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Pergola Calculator — Timber, Fixings & Postcrete
Calculate all the timber, fixings and concrete you need for a garden pergola build.
Length along the longest side
Width (rafter span)
2.4m standard (≈1.8m above ground once 600mm is concreted in)
Distance between posts along length
300-500mm typical, 400mm most common
10% recommended for timber cuts
Battens across rafters (2-3 typical)
600mm standard for posts up to 2.7m
300mm (12 inches) standard hole diameter
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How We Calculate This
This calculator works out all the timber and fixings needed for a standard rectangular pergola with two rows of posts running along the length. Choose L-shaped to plan a pergola made of two rectangular legs that meet at a right angle: each leg is calculated with the same rules, then the orders are merged (see the L-shaped section below).
Posts
Posts per side = ceil(Length ÷ Post Spacing) + 1 (minimum 2)
Total posts = Posts per side × 2
We round up so no gap between posts ever exceeds your chosen spacing. Post Height is the overall length of the post timber. For concreted posts with the standard 600mm buried, a 2.4m post stands ≈1.8m above ground (the beams then sit on top, so headroom under the beam is ≈1.8m minus the beam depth). Bolt-down shoes bury nothing, so a 2.4m post stands the full ≈2.4m above the base plate — buy posts ≈600mm longer if you want the same finished height as a concreted build.
Beams and rafters
Beams: 2 × pergola length (one each side, sitting on posts)
Rafters = ceil(Length ÷ Rafter Spacing) + 1, each spanning the width. This is the exact number you install — your wastage allowance is applied only to the linear-metre timber total, not to the rafter count.
Cross members: an optional, user-set number of battens (default 3), each running the full length on top of the rafters — a design choice, not a calculated quantity.
Fixings
Each post is allowed 2 coach bolts for the beam connection (a typical notch-and-bolt allowance — no standard governs the exact count). If using Postcrete, bags are worked out from your actual hole size: post holes are augered round, so concrete fill = (π × radius² × depth) minus the post, divided by ~0.018 m³ per 20kg bag (Blue Circle Postcrete data sheet, December 2023). A standard 300mm-diameter × 600mm-deep hole gives 2.0 bags of fill — matching the current data sheet (2 bags for a 100mm post in a 30cm × 60cm hole) — which we round up per post to 3 whole bags, because Postcrete is a fast-set product mixed one hole at a time and a part-bag can't be carried to the next post. Total timber is given in linear metres with your chosen wastage allowance.
L-shaped pergolas
In L-shaped mode the pergola is treated as two rectangular legs meeting at a right angle: Leg A is the main run and Leg B joins one end of Leg A, like the foot of the letter L. Measure the legs as two rectangles that meet at the junction without overlapping. Each leg is calculated with exactly the rules above (two rows of posts along the leg's length, 2 beams per leg, rafters spanning the leg's width), and the post-spacing and rafter-spacing rules apply to each leg separately. The totals are then merged: the two posts at Leg B's junction end stand where Leg A's frame already provides posts, so 2 posts are deducted from the combined order and Postcrete follows the physical post count (2 fewer holes). Coach bolts are not reduced, because a shared post carries beam connections from both legs and each leg keeps its full 2-bolts-per-post allowance. Your cross-member count applies per leg. This assumes Leg B's beams are bolted into Leg A's frame at the junction; if you build the two legs as separate free-standing frames instead, add the 2 shared posts back along with their fixings and concrete.
Frequently Asked Questions
Pergola post holes should be at least 600mm (2 feet) deep and around 300mm (12 inches) wide. This gives enough depth for stability and is the usual recommendation for structures up to about 2.7m tall. A round 300mm-diameter × 600mm hole with a 100×100mm post holds roughly 0.036 m³ of concrete — the current Blue Circle Postcrete data sheet (December 2023) lists 2 bags (20kg each) for exactly this case (a square 100mm post in a 30cm × 60cm hole). Because Postcrete sets in 5–10 minutes and is mixed one hole at a time, our calculator rounds each post up to whole bags (so a 0.036 m³ hole comes out at 3 bags per post — you cannot carry a part-bag over to the next post). A narrower 200mm-diameter hole holds about 0.013 m³, roughly 1 bag per post. For taller or exposed pergolas, go 750mm deep (the data sheet lists 2½ bags for a 30cm × 75cm hole) — the calculator works the bags out from the exact hole size you enter.
Standard pergola posts are 100×100mm (4×4 inches) treated softwood. Beams running along the length are typically 150×50mm or 200×50mm. Rafters spanning the width are usually 150×50mm or 200×50mm depending on the span — use 200×50mm for spans over 3m. All timber should be pressure treated (UC4 rated) for outdoor use.
Most garden pergolas fall under permitted development rights and don't need planning permission, provided they are: not on the front of the property facing a highway, not higher than 2.5m if within 2m of a boundary, and don't cover more than 50% of the garden. However, if you're in a conservation area or the property is listed, you should check with your local planning authority.
Pergola rafters are typically spaced at 300-500mm centres, with 400mm being the most common for a balanced look. Closer spacing (300mm) provides more shade but uses more timber. Wider spacing (500-600mm) gives a more open, airy feel. Our calculator defaults to 400mm but you can adjust this to suit your design.
The most common method is to notch the top of the posts and bolt the beams in place using M10 or M12 coach bolts (2 per connection). Use galvanised or stainless steel coach bolts for weather resistance. Alternatively, you can use galvanised joist hangers or decorative post-to-beam brackets. For decking next to your pergola, see our Decking Calculator.
Choose L-shaped as the pergola shape, then split your L into two rectangles: Leg A is the main run, and Leg B joins one end of Leg A at a right angle, like the foot of the letter L. Measure each leg up to where they meet, without overlapping, and enter each leg's length (along its beams) and width (the rafter span). The calculator works each leg out as its own rectangular pergola, then merges the order: the two posts at Leg B's junction end are shared with Leg A's frame, so 2 posts and their post holes are deducted, while coach bolts are not reduced because a shared post carries beam connections from both legs. You can plan other complex shapes (U-shapes, T-shapes and so on) the same way: split the outline into rectangles, run each one through the calculator, add the totals together and subtract 2 posts at every junction where one leg butts into another. As with any pergola, confirm rafter sizes with your timber supplier for spans over 3m.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.