Formula
How this estimate works
rafter length = square root of rise squared plus run squared
A 6 in rise and 12 in run gives a unit rafter length of about 13.42 in.
How rafter length is calculated
Rafter length is the hypotenuse of a right triangle formed by roof rise and horizontal run. The calculator uses the Pythagorean theorem.
Planning with rafter length
The calculated length is a geometric estimate. Birdsmouth cuts, overhangs, ridge details, and framing layout can change actual lumber lengths.
Assumptions used
- Pitch is shown as inches of rise per 12 inches of horizontal run.
- Rafter length uses the same units entered for rise and run.
- Pitch multiplier estimates sloped surface area from flat footprint area.
Before you order materials
- Use horizontal run, not sloped roof length.
- Add overhangs, ridge details, and cuts separately.
- Keep rise and run in the same units.
Frequently asked questions
Does this include roof overhang?
No. Add overhang and framing details separately.
Can I use inches or feet?
Use consistent units for rise and run. The result uses the same unit basis.
Is rafter length the same as roof run?
No. Run is horizontal distance; rafter length follows the slope.