Pergola Materials Calculator
Calculate posts, beams, rafters, and shade slats for a pergola.
Enter length, width, rafter and slat spacing for a full lumber list and board-foot total.
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 = (⌈Length ÷ 8 ft⌉ + 1) × Number of post rows Post rows = 2, plus another row each time the width passes a 12 ft rafter span Beam boards = Post rows × plies (2 when the beam is doubled) Rafters needed = ⌈Pergola Length ÷ Rafter Spacing⌉ + 1 Lumber board feet = Σ(Piece Count × Thickness in inches × Width in inches × Length in feet ÷ 12)
What each variable means:
- Posts: vertical supports. Typical size: 6×6 lumber for 8–12 ft heights. The calculator keeps them no more than 8 ft apart along the length, which is the conservative end of common practice.
- 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 span:
Member sizes follow the longest span, not the overall footprint. A big pergola with an extra post row can use lighter lumber than a small one built on four corner posts.
| Longest span between supports | Post Size | Beam Size | Rafter Size |
|---|---|---|---|
| Up to 8 ft | 4×4 | 2×8 single | 2×6 |
| 8 to 12 ft | 6×6 | doubled 2×10 | 2×8 |
| Over 12 ft | 6×6 | doubled 2×12 | 2×10 |
Worked example: A freestanding 12×16 ft pergola (12 ft wide × 16 ft long). Post spacing: 8 ft. Rafter spacing: 16 inches.
Posts: the 16 ft length splits into 2 bays of 8 ft, so 3 posts per side. The 12 ft width needs no intermediate row, so 2 rows. 3 × 2 = 6 posts, 8 ft apart along the length. Beams: one beam line per post row, doubled, each running the full 16 ft length = 4 pieces of 2×10. Rafters span the 12 ft width and are spaced 16 in apart along the 16 ft length: 192 in ÷ 16 in + 1 = 13 rafters, each 12 ft long. Slats run the 16 ft length and are spaced 4 in apart across the 12 ft width: 144 in ÷ 4 in + 1 = 37 slats, each 16 ft long.
Lumber list summary:
- 6×6 posts (8 ft): 6
- 2×10 × 16 ft beam boards: 4
- 2×8 × 12 ft rafters: 13
- 2×2 × 16 ft top slats: 37
Board feet: posts 6 × (6×6×8 ÷ 12) = 144, beams 4 × (2×10×16 ÷ 12) = 107, rafters 13 × (2×8×12 ÷ 12) = 208, slats 37 × (2×2×16 ÷ 12) = 197. Total ≈ 656 board feet.
At $30 per post the calculator scales the other pieces from that price (beams 0.83×, rafters 0.60×, slats 0.17×) and adds 15% for brackets, screws, and post anchors, landing near $808 in materials before concrete.
How we build and check this calculator
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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