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Rafter Calculator
Solve rafter length from rise and run, or work backwards from a known length. This is geometry, not a truss or load calculator.
Solve rafter geometry
Find length from rise and run, or work backwards from a known rafter.
Measurements stay in your browser
Rafter calculator FAQ
Three ways to solve the same triangle: length, rise, or run.
How do I calculate rafter length?
Rafter length is the hypotenuse of rise and run: √(run² + rise²). For a 6/12 pitch, every 12 inches of run needs about 13.4 inches of rafter.
Can I solve for rise or run instead?
Yes. If you know rafter length and run, rise = √(length² − run²). If you know length and rise, run = √(length² − rise²). Length equal to run is a flat roof (rise 0). Length equal to rise would be run 0, which this tool rejects.
Is this a truss or structural calculator?
No. This page is roof geometry only. It does not size lumber, check loads, or design trusses.