Rafter Length Calculator
Find the sloping reference length of one right triangle using horizontal run and either rise or pitch.
Geometry Tool 2 of 6 · Output: Sloping reference length
2Calculate & check
Enter measurements to see the result and its calculation schedule.
3Limits
This solves a single reference triangle. It does not adjust for ridge thickness, birdsmouth, member depth or cuts, and it does not determine the usable span or strength of timber. Use the Rafter Calculator for the separate span, ridge and overhang workflow.
How to use this calculator
- Select feet and inches or a metric unit.
- Enter the horizontal run, then choose whether you know the vertical rise or the x:12 pitch.
- Calculate to see the sloping length, rise, pitch and angle. Use the drawing to check your measurement endpoints.
Formula & method
R is horizontal run, H is vertical rise and L is the sloping reference length. Use consistent units for R and H. Pitch p describes rise per 12 horizontal units.
A worked example
Example inputs, not recommendations
A 12 ft horizontal run and a 5 ft rise produce a 13 ft sloping length. The same triangle has a 5:12 pitch and an angle of approximately 22.6199°.
Assumptions & limitations
Assumes
- The roof segment is straight and its rise and run form a right triangle.
- All entered measurements use the same endpoints. Any intended overhang is already represented in the triangle.
Does not cover
This solves a single reference triangle. It does not adjust for ridge thickness, birdsmouth, member depth or cuts, and it does not determine the usable span or strength of timber. Use the Rafter Calculator for the separate span, ridge and overhang workflow.
Geometric planning estimate. This does not size structural members or verify loads, connections, or code compliance. Verify structural and code requirements against local code or with a qualified professional.
Displayed values are rounded; subsequent calculations retain full precision. Numerical limits and accepted formats are documented in Calculation methods.
Common questions
Should I enter full building width?
Enter horizontal run, not full building span. For a symmetric gable, each slope covers roughly half the building width; ridge-face adjustments belong in the Rafter Calculator.
Can I include an overhang?
Yes, if both run and rise describe the complete intended segment. Increasing run while keeping the old rise changes the slope. For a separate eave allowance, use the Rafter Calculator.
Should I use rise or pitch?
Use the value you measured. Rise is a vertical distance for the entered run. Pitch is a ratio that stays the same for any segment along the same straight slope.