Construction
Instant, private, and free
Rafter Length Calculator.
Calculate rafter length, pitch, and angle from run and rise.
Set your values
Results update as you type.
Results use only the stated formula and entered dimensions; no material density, pricing, code, or structural recommendation is inferred.
Use Cases
Roof framing for a new shed or house
Determine the rafter length needed for a simple gable roof by entering the run and rise. This helps in cutting rafters accurately and estimating lumber requirements.
Example: For a shed with a run of 8 ft and a rise of 4 ft, the rafter length is about 8.94 ft.
Checking roof pitch for design or renovation
Use the calculator to find the pitch (rise over run) and angle of an existing roof, which is useful for matching new construction or installing solar panels.
Example: If run is 12 ft and rise is 6 ft, pitch is 6/12 and angle is about 26.57°.
Frequently Asked Questions
- How do I calculate rafter length using this calculator?
- Enter the horizontal run (the horizontal distance from the wall to the ridge) and the rise (the vertical height from the top of the wall to the ridge) in the same units. The calculator will compute the rafter length, pitch, and angle.
- What is the difference between run and rise?
- Run is the horizontal distance from the outside wall to the ridge (half the building width). Rise is the vertical distance from the top of the wall to the ridge. Both must be in the same units for accurate results.
- Can I use this calculator for any roof type?
- This calculator is designed for simple gable roofs where the rafter forms a right triangle with run and rise. For complex roofs (hips, valleys, or overhangs), additional calculations are needed.
Tips & Common Mistakes
Tips
- Ensure both run and rise are in the same units (e.g., feet, inches, meters) to get accurate results.
- For a symmetrical gable roof, the run is half the building width, measured from the outside wall to the ridge.
- Remember that the calculated rafter length is the theoretical length from the ridge to the wall plate; you may need to add overhang or subtract ridge thickness.
- Use the pitch and angle outputs to communicate with suppliers or contractors about your roof specifications.
Common Mistakes to Avoid
- Using the full building width instead of the run (half the width) for a gable roof, which doubles the rafter length.
- Mixing units, such as entering run in feet and rise in inches, without converting to the same unit.
- Forgetting that the rise is measured from the top of the wall plate to the ridge, not from the ceiling or floor.
Last updated: August 13, 2026