Rafter Length Calculator
Enter your roof's rise and run in inches to get the rafter line length, in both inches and feet.
What your result means
This is the rafter line length, the straight diagonal from the bird's mouth heel to the ridge. It does not include the overhang past the wall, so add that separately (the full Rafter Calculator handles overhang, angle, and pitch). Buy your lumber 1 to 2 ft longer for the cuts.
How to use this calculator
- Enter rise (height) and run (horizontal distance) in inches.
- Read rafter length in inches and feet.
- This is the LINE length only - add overhang separately.
- Add 6-12 in to buying length for trim cuts.
The formula
It's the Pythagorean theorem: the rafter is the hypotenuse, so square the rise and the run, add them, and take the square root. Divide by 12 for feet.
Worked example
Say your rise is 48 inches over a 144 inch run. The rafter line length is the square root of 48 squared plus 144 squared, which is 151.79 inches, or 12.65 ft. Add your overhang and a bit extra for the cuts.
Rafter line length by pitch (per 12 in of run)
| Pitch | Length per 12 in run |
|---|---|
| 4:12 | 12.65 in |
| 6:12 | 13.42 in |
| 8:12 | 14.42 in |
| 12:12 | 16.97 in |
Tips & gotchas
- This is the rafter line length, the diagonal from heel to peak.
- Add the overhang projection separately; the full Rafter Calculator does it for you.
- Buy lumber 1 to 2 ft longer than the number here for trim cuts.
- Snap chalk lines for the plumb cut and the bird's mouth.
- For the overhang, the added length is the overhang divided by the cosine of the roof angle.
Frequently asked questions
What is rafter length?
The diagonal distance from the bird's mouth heel up to the top of the ridge.
Does this include the overhang?
No. Use the full Rafter Calculator if you want the overhang figured in.
How is rafter length calculated?
The Pythagorean theorem: the square root of rise squared plus run squared.
What size lumber for a 12 ft rafter span?
Usually 2x8, or 2x10 for heavier loads or longer spans.
How accurate is this calculator?
The math is exact. Real-world cuts add maybe an eighth of an inch of tolerance.
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