Rafter Calculator
Calculate rafter length and angle for roof framing
Width of the building
Rise per 12" run
On-center spacing
Rafter Length
—
ft
Plumb Cut Angle
—
degrees
Number of Rafters
—
rafters
Ridge Board Length
—
ft
Lumber Cost
—
USD
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Our rafter calculator determines rafter length, plumb cut angles, and material quantities for your roof framing project.
- Measure building span
- Determine roof pitch
- Select rafter spacing
- Enter specifications
- Add 10% for waste and cuts
AI Insights
Smart analysis for your project
Smart Estimates
Our AI cross-references material costs, regional pricing, and project complexity for more accurate estimates.
Waste Optimization
AI-calculated waste factors based on project type, not flat percentages. Saves 5-15% on material costs.
Best Practices
Recommendations derived from building codes, contractor data, and thousands of completed projects.
Calculation Formula
Rafter length = (Span / 2) / cos(arctan(pitch/12))
Plumb cut angle = arctan(pitch/12) x (180/pi)
Rafters = (Ridge length / Spacing) + 1
Frequently Asked Questions
How do I calculate rafter length?
For a 6/12 pitch on a 24 ft span: rafter run = 12 ft, rise = 6 ft, rafter length = sqrt(12^2 + 6^2) = 13.42 ft per side.
What size lumber for rafters?
2x6 for spans up to 10 ft, 2x8 for 10-15 ft, 2x10 for 15-20 ft, 2x12 for 20+ ft. Check IRC span tables.