Pergola or Shade Structure Material Calculator
Calculate the number of posts, beams, rafters, and optional shade cloth needed to build a pergola or shade structure.
Formulas Used
Posts per side (length): ⌊Length ÷ Post Spacing⌋ + 1
Posts per side (width): ⌊Width ÷ Post Spacing⌋ + 1
Total Posts: 2 × (Posts Along Length + Posts Along Width) − 4 (corners shared)
Post Board-Feet: Total Posts × Post Height × Waste Factor
Beam Board-Feet: 2 × (Length + Width) × 2 (doubled) × Waste Factor
Number of Rafters: ⌊Length ÷ (Rafter Spacing in ft)⌋ + 1
Rafter Board-Feet: Number of Rafters × Width × Waste Factor
Shade Cloth (sq ft): Length × Width × Waste Factor
Waste Factor: 1 + (Waste % ÷ 100)
Assumptions & References
- Beams are doubled (two boards sistered together) on all four perimeter sides for structural integrity.
- Rafters span the width of the structure and are spaced at the specified on-center distance along the length.
- Posts are placed at the specified spacing along both the length and width perimeter; corner posts are counted once.
- One 60 lb bag of concrete per post footing is a standard residential rule of thumb for pergola posts (IRC Section R403).
- Post anchors (e.g., Simpson Strong-Tie ABA or ABU series) are assumed for each post base.
- Joist hangers (e.g., Simpson LUS or LUC series) are assumed at both ends of each rafter.
- Lag screws (3/8" × 3") are estimated at 4 per post-to-beam connection point.
- Shade cloth area equals the roof footprint; add extra for overlap and tie-down if needed.
- Board-feet are calculated as linear feet of lumber (actual dimensions not factored into board-foot count here; quantities are in linear board-feet for ordering purposes).
- References: IRC 2021 Chapter 5 (Floors), AWC Span Tables for Joists and Rafters, Simpson Strong-Tie Connector Catalog.