Roof measurement
Rafter length calculator
Find sloped rafter length. Homeowner has rise and run for a simple roof section.
Try the calculation
Enter rise and run for one roof section.
Adjustable example. Change these illustrative inputs to match your task, then recalculate. Reset restores the example and its result.
Understand the result
How this calculation works
Rafter length is the hypotenuse of one roof triangle.
Steps
- Measure the horizontal run and vertical rise for the same section.
- Enter both in feet and submit.
The form starts with the worked example below. Change its values and selections to match your task; Reset restores the example.
Formula
L = √(rise² + run²).
Variables and units
- Rise (ft)
- Required. Use a positive value. Starting example: 6. Replace it for your task.
- Run (ft)
- Required. Use a positive value. Starting example: 12. Replace it for your task.
What the result means
Use the value for a straight centerline estimate. Birdsmouth cuts, overhangs and structural design are outside this result.
Worked example
Inputs: Rise = 6; Run = 12.
Calculation: A 6 ft rise and 12 ft run gives √180 = 13.4164 ft, displayed as 13.42 ft.
Result: 13.42 ft.
Frequently asked questions
Is run half the building width?
For a symmetric gable section, it is usually the horizontal half-span used by the calculation.
Does this include overhang?
No. Add any overhang separately if your material plan requires it.
Can I use inches?
Convert measurements to feet before entering them.