How deep should pergola post holes be?

Pergola post holes should be at least 600mm (2 feet) deep and approximately 300mm (12 inches) square. This provides sufficient depth for stability and is the standard recommended for structures up to 2.7m tall. Each post hole uses approximately 1 bag of Postcrete (20kg). For taller or exposed pergolas, consider going 750mm deep.

What size timber do I need for a pergola?

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.

Do I need planning permission for a pergola?

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.

What spacing should pergola rafters be?

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.

How do I attach pergola beams to posts?

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 our Decking Calculator, see /landscaping/decking-calculator.

Pergola Calculator — Timber, Fixings & Concrete UK

Pergola Calculator — Timber, Fixings & Postcrete

Calculate all the timber, fixings and concrete you need for a garden pergola build.

Pergola Length

Length along the longest side

Pergola Width

Width (rafter span)

Post Height

2.4m standard (gives ~2.1m clear)

Post Spacing

Distance between posts along length

Rafter Spacing

300-500mm typical, 400mm most common

Rafter Timber Section

200×50mm for spans over 3m

Post Fixing Method

Postcrete in ground or bolt-down shoes

Wastage Allowance

10% recommended for timber cuts

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.

Posts

Posts per side = floor(Length ÷ Post Spacing) + 1 (minimum 2)
Total posts = Posts per side × 2
Standard post height is 2.4m (gives ~2.1m headroom after beam depth and 600mm in the ground for concreted posts, or full 2.4m clear for bolt-down shoes).

Beams and rafters

Beams: 2 × pergola length (one each side, sitting on posts)
Rafters = floor(Length ÷ Rafter Spacing) + 1, each spanning the width
Cross members: 3 × pergola length (running perpendicular to rafters)

Fixings

Each post needs 2 coach bolts for beam connections. If using postcrete, each 600mm deep × 300mm square post hole needs 1 bag of Postcrete (20kg). Total timber is calculated in linear metres with your chosen wastage allowance.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Pergola Calculator — Timber, Fixings & Postcrete

How We Calculate This

Posts

Beams and rafters

Fixings

Frequently Asked Questions

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