Project inputs
Example data table
| Inputs | Value | Typical output |
|---|---|---|
| Length × width × height | 12 ft × 10 ft × 8 ft | Top area ≈ 143 ft² (with 0.5 ft overhangs) |
| Layout + post spacing | Perimeter, 6 ft length spacing | 8 posts, 2 beam lines |
| Rafter + purlin spacing | 1.33 ft rafters, 2 ft purlins | ~11 rafters, 6 purlin rows |
| Waste factor + stock length | 10% waste, 12 ft stock | Board counts rounded up for planning |
| Shade coverage | 70% | Shade area ≈ 100 ft² |
Formula used
- Effective size: Le = L + 2×OL and We = W + 2×OW
- Post counts (perimeter): nL = ceil(Le/PSL)+1, nW = ceil(We/PSW)+1, posts = 2×nL + 2×(nW−2)
- Beam length: beam lines × Le × (single or double)
- Rafter count: floor(Le/RS)+1, rafter length ≈ We
- Purlin rows: floor(We/PuS)+1, each row length ≈ Le
- Waste factor: lumber totals × (1 + waste%/100)
- Shade area: Le×We×(shade%/100)
- Wind screening: wind pressure × (Le×H) (simplified projection)
How to use this calculator
- Pick units and enter pergola length, width, and height.
- Add overhangs to match the shade you want.
- Select a post layout and choose realistic spacing.
- Set rafter and purlin spacing for plants or panels.
- Adjust waste and stock length for your supplier.
- Optional: add pricing to estimate a project subtotal.
- Press calculate and download CSV or PDF summaries.
Dimensional planning and overhang effects
Using pergola length (L), width (W), and overhangs (OL, OW), the calculator first builds an effective footprint: Le = L + 2×OL and We = W + 2×OW. This matters because material and finish needs scale with Le×We. For example, a 12 ft by 10 ft frame with 0.5 ft overhangs becomes 13 ft by 11 ft, increasing top area to about 143 ft². If you plan a corner cutout, measure clear span to rail posts before choosing overhangs.
Post layouts and spacing choices
Post quantity depends on the support pattern you select. Corner mode fixes four posts. Perimeter mode adds posts along the long edges with nL = ceil(Le/spacing)+1, then wraps the short edges with nW. Grid mode multiplies nL×nW for row-and-column support when you need shorter beam spans or heavier plant loads. Many deck builds target 6 ft spacing to balance openness and strength.
Rafter and purlin density for shade
Rafter density is driven by rafter spacing (RS). The estimate uses floor(Le/RS)+1 rafters, each spanning We. Purlins run perpendicular and are counted by floor(We/PuS)+1 rows. Tightening RS from 24 in to 16 in can add several rafters on a mid-size pergola, improving shade and stiffness while raising cost. Dense purlins suit grapes, jasmine, and fabric shade panels.
Waste factor and stock-length rounding
Field cutting always produces offcuts, so totals are multiplied by a waste factor of 1 + waste%/100. Stock length then converts linear totals into rounded board counts by ceiling division, so the output reflects what you actually buy. If your supplier only stocks 10 ft boards, expect noticeably higher counts than with 12 ft stock. A 10% waste setting is common for square, repeatable cuts.
Cost drivers and export-ready summaries
Costs are estimated from unit prices for lumber, hardware, finish area, and optional shade coverage. Hardware scales with posts and rafter ends, while screw boxes scale with purlin-by-rafter intersections. The CSV and PDF exports capture the same quantities, making it easy to share assumptions and revise inputs as your deck measurements tighten. Always verify deck attachment, uplift resistance, and local design wind values.
FAQs
1) How should I choose a post layout on an existing deck?
Use corner layout for small spans and light shade. Choose perimeter when you can anchor along edges. Pick grid when beams would otherwise span too far or when vines, panels, or wind exposure demand more support points.
2) Does changing units convert my values automatically?
No. Switching units updates labels only. Enter measurements in the unit system you selected to keep totals correct.
3) What rafter spacing is typical for comfortable shade?
Common rafter spacing is 16–24 inches (0.4–0.6 m). Tighter spacing improves shade and stiffness but increases lumber and hanger counts.
4) Why do board counts look higher than linear totals suggest?
Boards are rounded up to whole stock lengths after adding waste. Offcuts and cutting patterns can’t be purchased as fractions, so the calculator reflects buying full boards.
5) How accurate is the wind force number?
It is a simplified screening estimate based on wind pressure times projected area. Use local design wind values and an engineered method for final sizing and connections.
6) What should I check before mounting a pergola on a deck?
Confirm joist direction, beam/ledger capacity, and attachment hardware. Ensure post bases resist uplift and lateral loads. If the deck is older, inspect fasteners and framing for rot or corrosion.
Notes for garden use
- Increase purlin density for heavy vines like grapes or wisteria.
- Use corrosion-resistant connectors near irrigation and coastal air.
- Plan a drip edge or gutter if shade panels shed water.
- Allow airflow and sun angles to support healthy plant growth.