Rafter Calculator
Calculate the rafter length and angle for a roof from its run, rise, and eave overhang.
- Free to use
- Accurate results
- No registration required
- Works on all devices
Enter your roof's run, rise, and eave overhang to find the rafter length and angle.
Horizontal distance from the wall to the ridge (usually half the building width).
Your result
4.572
Rafter length
Rafter angle20.56
AI explanation
Formula
Rafter length = √(run² + rise²) + overhang; angle = arctan(rise ÷ run)Worked example
4m run, 1.5m rise, 0.3m overhang
| Field | Value |
|---|---|
| Run (m) | 4 |
| Rise (m) | 1.5 |
| Eave overhang (m) | 0.3 |
| Rafter length | 4.572 |
| Rafter angle | 20.56 |
Assumptions
- Gives the raw rafter length only — actual lumber needs extra length for birdsmouth cuts and ridge connections, not included here.
Frequently asked questions
What counts as "run" for a rafter?
The horizontal distance from the outer wall to the ridge — typically half the building's width for a simple gable roof.
Why add the overhang?
The rafter usually extends past the wall line to form the eave, which protects the wall from rain — the overhang length is added to the structural (wall-to-ridge) rafter length.
How do I find the roof's slope angle if I don't know rise and run?
Use the Roof Pitch Calculator, or check your roof design drawing for the specified pitch.