How to use the pergola calculator
A pergola is a simple frame: posts carry beams, beams carry rafters, and optional top slats (purlins) run across the rafters for extra shade. This calculator lays that frame out on your footprint and gives you a piece count and length for each member, so you can draw it, price it or take it to the lumber yard.
- Enter the length along the beams and the width between beam lines. The beams run along the length; the rafters span the width.
- Set the maximum post spacing (often 8–10 ft or 2.4–3 m for timber) and the number of beam lines, normally 2.
- Add overhangs for the beam ends and the rafter tails. A 12–24 in (300–600 mm) overhang is typical and looks lighter.
- Choose rafter and slat spacing, plus the rafter and slat thickness for the shade estimate.
- Enter post height, burial depth and hole size for the post length and concrete, or set the depth to 0 for surface-mounted post bases.
Pergola formulas
Posts per beam line = ⌈length ÷ max spacing⌉ + 1 · posts = posts per line × beam lines
Beam length = length + 2 × beam overhang · beam boards = beam lines × plies
Rafters = ⌈beam length ÷ rafter spacing⌉ + 1 · rafter length = width + 2 × rafter overhang
Slats = ⌈rafter length ÷ slat spacing⌉ + 1 · slat length = beam length
Concrete per hole = π × (hole ÷ 2)² × depth − post² × depth
Shade ≈ 1 − (1 − rafter thickness ÷ rafter spacing) × (1 − slat thickness ÷ slat spacing)
Connectors are counted as one post-to-beam connector per post and one rafter tie wherever a rafter crosses a beam line. The shade figure is for the sun straight overhead and counts only the thickness of each member, so it is a rough lower bound.
Worked examples
16 × 12 ft backyard pergola
With posts at most 10 ft apart: ⌈16 ÷ 10⌉ + 1 = 3 posts per side, 6 posts. Beams are 16 + 2 × 1 = 18 ft long, so 4 boards for two doubled beams, each needing a splice or 18 ft stock. Rafters at 16 in: ⌈216 ÷ 16⌉ + 1 = 15 rafters, each 12 + 2 = 14 ft. Slats at 6 in across a 14 ft rafter: ⌈168 ÷ 6⌉ + 1 = 29 slats, 18 ft long. Six 12 in holes 3 ft deep with 6×6 posts take about 10.4 ft³ of concrete, or 18 bags of 80 lb mix before waste.
Metric 4 × 3 m pergola
Posts at most 3 m apart: ⌈4 ÷ 3⌉ + 1 = 3 per side, 6 posts. With 300 mm overhangs, beams are 4.6 m and rafters 3.6 m. Rafters at 400 mm give ⌈4,600 ÷ 400⌉ + 1 = 13 rafters.
Who this calculator can help
- Landscape architects and garden designers sketching outdoor rooms and checking post layouts against paving and planting.
- Architects adding a pergola to a terrace, roof deck or rear extension and needing a quick quantity check.
- Contractors and carpenters preparing a lumber list and quote.
- Homeowners and DIY builders pricing a pergola kit against building from stock lumber.
Practical tips
- Keep the timber out of the ground where you can: a post base on a concrete pier lasts longer than a buried post, even pressure-treated.
- Use stock lengths that suit the members. If a beam is longer than your longest board, plan the splice over a post.
- Brace posts with knee braces or rigid post bases in windy areas; tall slender pergolas rack sideways without them.
- Check rafter spans with the joist and rafter span calculator, footings with the concrete calculator, and price the timber with the board foot calculator.
- Building a deck under it? The deck calculator counts boards and joists, and the sun path calculator shows where the shade will fall.
Want to see the pergola in your garden first? Try a few designs on a photo with AI Landscape Design.
