Rafter Length Calculator
Find common-rafter slope length including horizontal overhang.
Loading calculator…
What this tool does
Find common-rafter slope length including horizontal overhang. Ridge deductions, birdsmouth and end cuts are not subtracted.
How to use the Rafter Length Calculator
- Enter or select horizontal rafter run in ft.
- Enter or select roof pitch (rise per 12).
- Enter or select horizontal overhang in ft.
- Read the calculated result; change any measurement to compare alternatives.
Formula
rafter=(run+overhang)×sqrt(1+(pitch/12)²)
- run
- Horizontal rafter run (ft, before metric conversion)
- pitch
- Roof pitch (rise per 12)
- overhang
- Horizontal overhang (ft, before metric conversion)
Ridge deductions, birdsmouth and end cuts are not subtracted.
Worked example
For rafter length calculator, the following measurements illustrate the exact method: Horizontal rafter run: 10; Roof pitch (rise per 12): 6; Horizontal overhang: 1.
Inputs
- Horizontal rafter run10 ft
- Roof pitch (rise per 12)6
- Horizontal overhang1 ft
Result
- Geometric rafter length12.3 ft
- Rise over run5 ft
Results explained
- Geometric rafter length
- Geometric rafter length from the formula above, expressed in ft before metric conversion. Ridge deductions, birdsmouth and end cuts are not subtracted.
- Rise over run
- Rise over run from the formula above, expressed in ft before metric conversion. Ridge deductions, birdsmouth and end cuts are not subtracted.
Frequently asked questions
Use Pythagoras with horizontal run and rise derived from pitch.
Enter horizontal run, pitch and horizontal overhang, not slope length.
Use 5–10% for simple layouts and 10–15% for diagonal cuts or complicated outlines; the allowance is editable.
Split the outline into rectangles or triangles and calculate each section separately. Do not count overlapping sections twice.
This is a purchasing estimate, not a structural design. Confirm measurements and local requirements before buying or building.