Pergola Calculator
Calculate materials needed for a pergola — posts, beams, rafters, and hardware. Enter values for instant results with step-by-step formulas.
Formula
Rafters = (Length / Rafter Spacing) + 1 | Post Length = Height + 3ft burial depth
Rafter count is calculated by dividing the pergola length by the rafter spacing in feet and adding one. Beam length includes overhang on each end. Post length includes 3 feet of burial depth for structural stability.
Worked Examples
Example 1: Standard Backyard Pergola
Problem: Calculate materials for a 12 x 10 foot pergola, 9 feet tall, with 4 posts, 16-inch rafter spacing, and 12-inch cross beam spacing.
Solution: Posts: 4 (6x6, 12 ft long for 9ft height + 3ft burial)\nBeams: 2 main beams, 14 ft each (12ft + 2ft overhang)\nRafters: (12ft / 1.33ft) + 1 = 10 rafters, 12ft each\nCross beams: (10ft / 1ft) + 1 = 11 purlins\nConcrete: 4 posts x 2 bags = 8 bags
Result: 4 posts, 2 beams, 10 rafters, 11 cross beams, 8 concrete bags
Example 2: Large Entertainment Pergola
Problem: Calculate for a 16 x 14 foot pergola, 10 feet tall, 6 posts, 12-inch rafter spacing, 12-inch cross beam spacing.
Solution: Posts: 6 (6x6, 13 ft long)\nBeams: 2 main beams, 18 ft each\nRafters: (16ft / 1ft) + 1 = 17 rafters, 16ft each\nCross beams: (14ft / 1ft) + 1 = 15 purlins\nConcrete: 6 posts x 2 bags = 12 bags
Result: 6 posts, 2 beams, 17 rafters, 15 cross beams, 12 concrete bags
Frequently Asked Questions
What size lumber should I use for a pergola?
Pergola lumber sizes depend on the span and load requirements of each component. Posts should be at least 6x6 inches for freestanding pergolas to provide adequate structural support and visual proportion. Main beams are typically 2x8, 2x10, or doubled 2x6 boards depending on the span between posts. For spans up to 10 feet, 2x8 beams are sufficient. For 10 to 14-foot spans, use 2x10 or doubled 2x8 beams. Rafters are commonly 2x6 or 2x8 boards spaced 12 to 24 inches apart. Top lattice or purlins can be smaller 2x2 or 1x2 boards since they carry minimal load. Always check local building codes for specific structural requirements in your area.
How deep should pergola posts be buried?
Pergola posts should be buried at least 3 feet deep for a standard 9 to 10-foot tall pergola, or one-third of the total post length. In areas with frost, posts must extend below the frost line to prevent heaving, which can range from 30 to 48 inches depending on your climate zone. The post hole diameter should be 12 to 16 inches, approximately three times the post width. Fill the bottom 6 inches of the hole with gravel for drainage, then set the post in concrete. Each post hole typically requires 2 to 3 bags of 50-pound concrete mix. Use a post level to ensure the post is perfectly plumb in both directions before the concrete hardens, and brace the post temporarily for at least 24 hours.
How far apart should pergola rafters be spaced?
Pergola rafter spacing typically ranges from 12 to 24 inches on center, depending on the desired shade coverage and aesthetic preference. Closer spacing of 12 to 16 inches provides more shade and a denser visual appearance. Wider spacing of 18 to 24 inches creates an airier, more open feel with less shade. The structural consideration is that rafters must support their own weight plus any loads from climbing plants, hanging lights, or retractable shade canopies. For 2x6 rafters spanning up to 12 feet, 16-inch spacing is the most common choice that balances shade, appearance, and structural performance. If you plan to grow heavy vines like wisteria, use closer spacing and larger rafters.
Do I need a permit to build a pergola?
Permit requirements for pergolas vary significantly by municipality and depend on the pergola size, location, and whether it is attached to the house or freestanding. Many areas require a building permit for any structure over a certain square footage, commonly 120 to 200 square feet. Attached pergolas that connect to the house almost always require a permit and may need to meet specific structural requirements to avoid damaging the existing structure. Even if a permit is not required, you must comply with property setback requirements that dictate minimum distances from property lines. Contact your local building department before starting construction and have your plans reviewed to avoid costly compliance issues later.
What is the best wood for a pergola?
The best wood species for pergolas depends on your budget, aesthetic preference, and maintenance willingness. Pressure-treated pine is the most economical option at 2 to 4 dollars per linear foot for structural members, with a lifespan of 20 to 30 years. Western red cedar is naturally rot and insect resistant with a beautiful reddish-brown color, costing 4 to 8 dollars per linear foot. Redwood is the premium choice with exceptional durability and appearance but costs 6 to 12 dollars per linear foot and is difficult to source outside the western United States. For the lowest maintenance option, consider composite or vinyl pergola kits, though they cost significantly more at 3,000 to 10,000 dollars for a complete structure.
How much shade does a pergola provide?
A standard pergola with rafters spaced 16 inches apart provides approximately 50 to 60 percent shade coverage, depending on the rafter width and the angle of the sun. At midday with the sun directly overhead, shade is at its maximum because sunlight must pass between the narrowest gaps. In the morning and evening when the sun is low, a pergola provides less shade as sunlight enters at an angle beneath the rafters. To increase shade coverage, add lattice panels, shade cloth rated at 70 to 90 percent blockage, retractable canopies, or grow climbing plants like grapevines, wisteria, or climbing roses. Adjustable louvered pergolas allow you to control shade from full sun to full coverage.