Pergola Calculator — Timber, Fixings & Postcrete

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.