Rafter Calculator
Find rafter length from building width, pitch and overhang.
Rafter length
13.42 ft
- With overhang
- 14.53 ft
- Run
- 12 ft
- Rise
- 6 ft
- Pitch angle
- 26.6°
Geometry only — this does not determine structural load capacity or code compliance. Confirm framing with a licensed professional.
Save This CalculationHow we calculate this
Run is half the building width. Rise = run × (pitch ÷ 12). Rafter length is found with the Pythagorean theorem: √(run² + rise²). Overhang extends the run before this calculation.
Example
Measuring tips
Measure building width at the exterior wall-to-wall span. Confirm your pitch from existing framing or your architectural plans — common residential pitches range from 4/12 to 8/12.
Common mistakes
- Using the full building width instead of half (the run) in the rise calculation.
- Forgetting to add overhang, resulting in rafters cut too short.
- Assuming this geometry calculation accounts for structural load — it does not.
Frequently asked questions
Does this calculator check structural requirements?
No — this is a geometry calculator only. Rafter sizing, spacing, and load capacity must be confirmed against local building code and, where required, a licensed engineer.
What is roof pitch?
Roof pitch is the rise in inches over a 12-inch run, written as X/12. Use our Roof Pitch Calculator to convert between rise/run and pitch notation.
Related calculators
Estimates are provided for planning purposes. Actual material requirements may vary based on supplier specifications, compaction, site conditions, installation method, waste, and local code. This calculator does not provide engineering certification or structural approval.