Start with the right measurement
A rafter length estimate starts with the horizontal half-span and follows the roof slope. It is useful for an early material takeoff, but it is not a finished cut list because ridge details and cuts change the piece you order.
How to use this calculator
- 01Run: Horizontal distance from wall plate to ridge, usually half the building span.
- 02Pitch: Rise in inches per 12 inches of run.
- 03Overhang: Horizontal projection beyond the wall, if needed.
The result is the sloped geometric line from the bearing point to the ridge or eave projection. It does not subtract the ridge board or add plumb, seat, or tail cuts.
Units: Use feet or metres consistently for run and overhang. Pitch is entered as rise per 12 horizontal units, such as 6 for 6:12.
How the calculation works
Rafter length = horizontal run × √(1 + (pitch ÷ 12)²). The overhang is added as a horizontal projection before applying the slope factor.A 12-foot run at 6:12 has a basic rafter length of about 13.42 feet before any cut allowance.
Worked examples
Simple gable
- Enter a 12 ft wall-to-ridge run and 6:12 pitch.
- The slope factor is √(1 + 0.5²) = 1.118.
- Multiply 12 by 1.118.
Result: The geometric length is about 13.42 ft.
Add a horizontal eave
- Use a 10 ft run, 4:12 pitch, and 1.5 ft horizontal overhang.
- The calculation run becomes 11.5 ft.
- Multiply 11.5 by √(1 + (4/12)²).
Result: The sloped estimate is about 12.14 ft; confirm the actual tail and cut lines in the field.
How to measure
- 01Enter the horizontal distance from wall plate to ridge—not the full building span.
- 02If you know full span, divide it by two before entering it.
- 03Measure any eave projection horizontally.
Common mistakes to avoid
- Entering full span instead of half-span
- Treating overhang as sloped length
- Ignoring field cuts and ridge details
Safety note
Roof framing is a fall and structural hazard. Use a qualified person for design, temporary bracing, and safe installation.
Limits of this estimate
Results exclude ridge-board deductions, birdsmouth cuts, lumber defects, loads, and structural requirements.
Frequently asked questions
Does overhang follow the slope?
Enter its horizontal projection; the calculator converts it to sloped length.
Is this a cut list?
No. It is a geometric estimate for planning.