What size beams for a pergola?

Doubled 2×10 or 2×12 for 10–13 ft spans as a rule of thumb.

Can I add a polycarbonate roof?

Yes, with at least a 5% slope and closer rafters.

Wood or aluminum pergola?

Wood is warmer and cheaper; aluminum needs almost no maintenance.

Pergola material calculator

Length, width, height, overhang and rafter spacing: you get posts, main beam footage, number and length of rafters, and post anchors.

Purchase quantity

4 post

Net measurement: 12 m²

Posts4 post
Post length2.4 m
Main beams (2)9.2 m
Rafters / joists9 unit
Rafter length3.6 m
Post anchors4 unit

Use my prices

Your prices per language/currency. Used when you enable “Use my prices” in calculators. My prices →

Calculate labour, margin & GST

How it's calculated (visible formula)

posts = 2 × (ceil(length ÷ 4 m) + 1); rafters = ceil(length ÷ spacing) + 1; rafter = width + 2 × overhang

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Set length, width and clear height.
Space posts with spans up to about 13 ft.
Work out main beams with overhang.
Work out rafters from spacing and length with overhangs.
Add post anchors, hardware and fasteners.

Worked example: 13 × 10 ft pergola, 8 ft tall, 12 in overhang, rafters 20 in OC

2 × (1 + 1) = 4 posts
Beams: 2 × (13 + 2) ft = 30 ft
13 ft ÷ 20 in ≈ 9 rafters of 12 ft

→ 4 posts, 30 ft of beam and 9 rafters of 12 ft.

Common mistakes

Spans over 13 ft without upsizing the beam.
No knee braces, so the pergola racks.
Setting wood posts in direct contact with soil.

Pro tips

Anchor posts with standoff bases on concrete piers so they don't touch soil.
Brace corners with 45° knee braces.
For attached pergolas, fasten a ledger to the wall with structural anchors.
Re-stain wood every 2–3 years.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

Create quote Calculate margin & GST

FAQs

What post size?: 6×6 is standard for 8–9 ft tall pergolas.
Do I need a permit?: Often for attached or large pergolas — check locally.
Rafter spacing?: 16–24 in; closer for shade cloth or polycarbonate.
What size beams for a pergola?: Doubled 2×10 or 2×12 for 10–13 ft spans as a rule of thumb.
Can I add a polycarbonate roof?: Yes, with at least a 5% slope and closer rafters.
Wood or aluminum pergola?: Wood is warmer and cheaper; aluminum needs almost no maintenance.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.