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
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.