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.**