Rafter Length Calculator
Calculate the geometric rafter length from the roof run and rise.
Rafter length
4.47m
- Roof angle
- 26.6 °
Geometric length only; it does not include overhang, birdsmouth cuts or structural design.
v1.0 · Reviewed September 2026
How this is worked out
The rafter is the hypotenuse of the run and rise, √(run² + rise²). The angle is the arctangent of rise ÷ run.
Worked example
- Horizontal run 4 m
- Rise 2 m
√(4² + 2²) = 4.47 m, and arctan(2 ÷ 4) = 26.6°
4.47 m at 26.6°
Assumptions and limits
- Geometric length only. Add overhang and allow for the birdsmouth and ridge cuts.
- Timber sizing and spacing are a structural matter and are not covered here.