Pergola Materials Calculator
Calculate posts, beams, rafters, and shade slats needed for a pergola.
Enter length, width, and height to get a complete lumber materials list and count.
A pergola is an outdoor garden structure with open lattice roofing, typically supported by vertical posts and horizontal beams and rafters. Calculating the materials — posts, beams, rafters, and lumber — correctly before purchasing prevents costly overruns and ensures structural integrity.
Core formulas: Posts needed = (Pergola Bays Long + 1) × (Pergola Bays Wide + 1) Beams needed = (Number of rows parallel to beam direction) × Beam Span Count Rafters needed = ⌈Pergola Length ÷ Rafter Spacing⌉ + 1 Lumber board feet = Σ(Piece Count × Length × Width × Thickness ÷ 12)
What each variable means:
- Posts: vertical supports. Typical size: 6×6 lumber for 8–12 ft heights. Spacing: 8–12 ft between posts.
- Beams: horizontal members spanning between posts, carrying the rafters. Typical size: 2×8 or 2×10, often doubled for spans over 10 ft.
- Rafters: smaller horizontal members spanning beam to beam. Typical size: 2×6, spaced 12–24 inches apart.
- Rafter spacing: closer spacing (12") gives a denser shade feel; wider spacing (24") gives an open, airy feel.
- Ledger board: if attached to a house, the ledger replaces the house-side beam posts.
Standard material sizes by pergola dimension:
| Pergola Size | Post Size | Beam Size | Rafter Size |
|---|---|---|---|
| Up to 10×10 ft | 4×4 or 6×6 | 2×8 single | 2×6 |
| 10×12 to 12×16 ft | 6×6 | 2×10 doubled | 2×8 |
| 16×20 ft+ | 6×6 or 8×8 | 3×12 or doubled 2×12 | 2×8 |
Worked example: A freestanding 12×16 ft pergola (12 ft wide × 16 ft long). Post spacing: 8 ft. Rafter spacing: 16 inches.
Posts: (16÷8 + 1) × (12÷8 + 1) = 3 × 2.5 → 3 × 3 = 9 posts (rounded to even bays: 3 along length, 3 across = 6 posts for two bays × two bays) Beams (spanning 12 ft): 2 beams, each 12 ft long, doubled (4 pieces of 2×10 × 12 ft) Rafters (along 16 ft run, 16" spacing): 16 ft ÷ (16÷12 ft) + 1 = 13 rafters, each 12 ft long
Lumber list summary:
- 6×6 posts (8 ft): 6
- 2×10 × 12 ft beams: 4
- 2×8 × 12 ft rafters: 13
Cost estimate: Standard pressure-treated lumber; 6×6×8: ~$30 each; 2×10×12: ~$25 each; 2×8×12: $18 each.
Rough material cost: (6×$30) + (4×$25) + (13×$18) = $180 + $100 + $234 = **$514 in lumber.**
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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