Rafter Length Calculator
Calculate the length of a rafter given the span, overhang, and pitch (slope). Perfect for roof framing projects.
Published by:
isCalculator Editorial Team
Was this calculator helpful?
How to use this tool?
How to Use the Rafter Length Calculator
- Enter the Span – The horizontal distance from wall to wall (e.g., 20). Must be a positive number.
- Enter the Overhang – The horizontal extension beyond the wall (e.g., 2). Use 0 if none.
- Enter the Pitch – The roof rise per 12 inches of run (e.g., 6 for a 6/12 pitch). Use 0 for a flat roof.
- Select Units – Choose Feet or Meters for all inputs and the result.
- Click “Calculate Rafter Length” – The result shows the rafter length in the chosen unit.
Formula used: Rafter Length = (Span/2 + Overhang) / cos(arctan(Pitch/12))
Tip: All inputs must be non-negative; Span must be greater than zero. The calculator accepts decimals for precision.
Previous Results
| Result | Inputs | Date |
|---|
Frequently Asked Questions
A rafter is a structural beam that extends from the ridge of a roof to the wall plate, supporting the roof covering.
Rafter length is calculated using the formula: (span/2 + overhang) / cos(arctan(pitch/12)), where pitch is the rise per 12 inches of run.
The calculator supports feet and meters. You can select the unit from the dropdown menu.