This rafter length calculator finds the true length of a common rafter from the roof’s run and rise. Enter your measurements above to get the rafter length, then add your overhang to size the board you cut.
How to use this calculator
- Enter the run — usually half the building span.
- Enter the rise — the height gained over that run (from the pitch).
- Read the rafter length, then add your overhang.
How is rafter length calculated?
The rafter is the hypotenuse of the triangle formed by rise and run:
Rafter length = √(Rise² + Run²) Total board = Rafter length + Overhang
- Run — horizontal distance, wall to ridge (feet)
- Rise — vertical height over that run (feet)
Example calculation
For a 24 ft span at a 6:12 pitch:
- Run = 24 ÷ 2 = 12 ft
- Rise = 12 × (6 ÷ 12) = 6 ft
- Rafter = √(6² + 12²) = √180 = 13.42 ft
- Add a 1 ft overhang → cut boards about 14.4 ft
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Frequently Asked Questions
How do I calculate rafter length?
Rafter length is the square root of rise squared plus run squared. For a 12 ft run and 6 ft rise, that is the square root of 180, or about 13.4 feet.
What is the run of a rafter?
The run is the horizontal distance from the wall to the ridge, usually half the building span for a symmetrical gable roof.
Does rafter length include the overhang?
The base calculation gives the length from wall to ridge. Add your desired overhang (eave) length separately to get the board you cut.