Wern-WoodFree calculator
Pergola Kit Calculator
Enter your pergola dimensions for an instant materials list — then add everything to your basket in one tap.
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Pergola dimensions
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Timber options
▼ Your materials estimate
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Posts
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Beams
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Rafters
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Post shoes
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ⓘ Quantities are rounded up to the nearest whole item. This is a guide — verify against your build before ordering.
Post size — which should you choose?
Posts are the uprights that carry the whole structure. Size affects strength, visual weight, and how the pergola feels.
75×75mm
3×3 inch
The lightest option. Fine for smaller, sheltered builds where the structure doesn't need to carry much weight.
- Spans up to around 2.5m
- Sheltered, enclosed gardens
- Lightweight overhead structure
- Budget or temporary builds
100×100mm
4×4 inch
The standard choice for most garden pergolas. Strong enough for typical spans, looks proportional, and is widely stocked at sensible prices.
- Spans up to around 3–3.5m
- Standard exposed gardens
- Most freestanding builds
150×150mm
6×6 inch
A substantial post for larger or more demanding builds. Noticeably heavier looking — which is often exactly what you want for a feature pergola.
- Spans of 4m or more
- Exposed or windy locations
- Exposed or windy locations
- Feature or statement builds
75×75mm
Cross section at scale relative to other sizes
100×100mm
Noticeably chunkier — the right choice for most builds
150×150mm
Substantial. Sets a different visual tone for the whole build
Beams — what they are and which size
Beams run along the length of the pergola, sitting on top of the posts. They carry the rafters and everything on the roof.
Beams run along the depth
One beam each side (freestanding), resting on top of the posts. The rafters then rest on top of the beams.
47×150mm
6×2 inch — structural timber
The go-to beam size for most garden pergolas. Plenty of strength for spans up to around 4m with normal overhead loads.
- Spans up to ~4m
- Most residential pergola builds
- Most residential pergola builds
- Pairs well with 100×100 posts
47×200mm
8×2 inch — structural timber
A deeper, stiffer beam for wider spans or heavier loads. Also gives a more architectural look if that suits the build.
- Spans of 4m or more
- Pairs well with 150×150mm posts
- Heavier roof infill or fabric
- Pairs well with 150×150 posts
Rafters — what they are and which size
Rafters span across the width, sitting on top of the beams. They're the visible "fingers" of the roof and set the visual rhythm of the structure.
Rafters span the width
They cross over the top of both beams and overhang each side by around 300mm. The spacing between them is your rafter spacing setting.
47×100mm
4×2 inch — structural timber
The standard rafter for most garden pergolas. Light enough to look elegant, strong enough for typical widths and typical overhead use.
- Widths up to ~3.5m
- Open, airy overhead feel
- Open, airy feel
- Standard residential builds
47×150mm
6×2 inch — structural timber
A deeper rafter for wider spans or heavier loads. Also adds a more substantial, architectural feel to the overhead structure.
- Widths of 4m or more
- Wider spans or heavier builds
- Where deflection over span matters
- When visual weight is the goal
Rafter spacing — 400mm vs 600mm centres
Spacing controls how many rafters you use, how much light gets through, and how much coverage you want overhead.
400mm centres
Closer spacing — more rafters
More rafters, more overhead coverage, The structure feels more solid with more coverage overhead.
- Exposed or windy gardens
- Where you want more overhead coverage
- More coverage and shade overhead
- A more substantial, solid look
600mm centres
Wider spacing — fewer rafters
The standard for most garden pergolas. Open, airy feel with plenty of light. Suits most standard garden pergola builds.
- Open, airy feel with more light through
- Sheltered gardens
- More light and airflow through the roof
- Most standard residential builds
Visual comparison at the same width
400mm feels denser and more sheltered. 600mm is open and airy. Neither is wrong — it depends on what the build is for.
Post fixing — bolt-down shoe vs in-ground postmix
How you fix the posts to the ground affects the build process, the post length you need, and the long-term durability.
Bolt-down shoe
Surface mount — onto concrete, slabs or patio
A metal shoe bolted to an existing solid surface. The post sits above ground level, which avoids the main cause of timber rot — prolonged contact with damp soil.
- Existing patio, concrete or paving
- Posts stay shorter (no burial depth needed)
- Post base stays dry — longer lifespan
- Easier to replace a post if needed
- Not available in 150×150mm
In-ground postmix
Set into the ground with rapid-set concrete
Posts go directly into the ground, set in postmix concrete. A strong, permanent fixing — but the post must be long enough to allow 600mm below ground.
- Grass, soil or gravel surfaces
- Posts need 600mm extra length for burial
- Very solid once set
- Required for 150×150mm posts
- Allow 24 hours before loading
On a patio or concrete?
Use bolt-down shoes. Drill into the surface and fix with coach bolts. Your posts can be shorter and the base stays dry. This is the cleaner install for most garden builds.
On grass or soil?
Use in-ground postmix. Add 600mm to your chosen height to get the right post length. Make sure posts are perfectly plumb before the postmix sets — you won't be moving them afterwards.
Freestanding vs lean-to
This changes how many posts you need and how the structure is supported.
Freestanding
4 posts — stands independently
The standard pergola — four posts carrying two beams and a set of rafters. Stands in the middle of a garden or on a patio with no attachment to the house.
- 4 posts, 2 beams, full rafter set
- Can go anywhere — no wall needed
- Fully self-supporting structure
- Corner braces on all four corners
Lean-to
2 posts — attached to a wall or building
One side of the pergola is supported by a ledger board fixed to a wall. You only need posts on the free-standing side. The calculator covers the 2 free posts — you'll need to fix a ledger board to the wall separately.
- 2 posts on the outer side only
- 1 beam on the outer side
- Ledger board fixed to wall (not included)
- Use appropriate fixings for your wall type
