How to calculate rafter length
Use the horizontal run plus overhang as one leg of a right triangle. The roof pitch gives the rise. Rafter length is the sloped hypotenuse of that triangle.
Run, pitch, and overhang
The default example uses 12 ft of horizontal run, a 6/12 pitch, and 1 ft of horizontal overhang. The formula calculates the sloped length before birdsmouth, plumb cuts, ridge details, or field trimming.
Length is not structural design
This page estimates geometry only. It does not size rafters, approve spans, select lumber grade, calculate loads, design trusses, or replace local code and plan requirements.
When to calculate separately
Calculate separate roof sections when run, pitch, overhang, or framing type changes. Hip, valley, jack, and irregular rafters need their own layout checks.
Rafter length by run and pitch
Examples do not include overhang, birdsmouth, ridge detailing, or field trimming. They are geometric length checks only.
| Horizontal run | 4/12 pitch | 6/12 pitch | 8/12 pitch |
|---|---|---|---|
| 8 ft run | 8.43 ft | 8.94 ft | 9.61 ft |
| 10 ft run | 10.54 ft | 11.18 ft | 12.02 ft |
| 12 ft run | 12.65 ft | 13.42 ft | 14.42 ft |
| 16 ft run | 16.87 ft | 17.89 ft | 19.23 ft |
Common roof pitch checks
Pitch is rise per 12 inches of run. Use plans or safe measurements; do not climb onto unsafe roofs for calculator inputs.
| Pitch | Approx. angle | Area multiplier |
|---|---|---|
| 4/12 | 18.4 degrees | 1.054x footprint |
| 6/12 | 26.6 degrees | 1.118x footprint |
| 8/12 | 33.7 degrees | 1.202x footprint |
| 12/12 | 45 degrees | 1.414x footprint |
Before you calculate
- Use horizontal run in feet, not the sloped roof length.
- Enter pitch as inches of rise per 12 inches of run.
- Add overhang as horizontal projection before calculating the sloped length.
Common mistakes
- Using rafter length as if it were a structural span approval.
- Forgetting that birdsmouth, ridge cuts, plumb cuts, and field trimming are separate.
- Applying one rafter length to roof sections with different run, pitch, or overhang.
Formula
rafter length = sqrt((run + overhang)^2 + ((run + overhang) * pitchRise / 12)^2)
Assumptions
- This is not a complete rafter layout.
- It excludes plumb cuts, seat cuts, and structural design.
- Verify dimensions against plans.
Example
Estimated rafter length: 14.53 ft
How to calculate rafter length
- Enter the horizontal run for the roof section.
- Enter roof pitch as rise per 12 inches of run.
- Add overhang when the calculator includes it in the sloped rafter length.
- Use the right-triangle relationship between run and rise to estimate the sloped rafter length.
- Check birdsmouth cuts, ridge details, lumber sizing, loads, and local framing requirements separately.
Before you buy materials
- Use the result as a layout check before selecting lumber lengths.
- Verify rafter size, spacing, species, grade, load, and code requirements separately.
FAQ
What is the example rafter measurements ft result?
Use horizontal run, pitch rise, overhang, then calculate the planning result. In the default example, the result is 14.53 ft.
What is the rafter length formula?
Use rafter length = square root of horizontal run squared plus rise squared. Rise equals horizontal run times pitch rise divided by 12.
Does rafter length include overhang?
On this page, yes. The overhang input is added to horizontal run before the sloped length is calculated.
Is run the same as rafter length?
No. Run is the horizontal distance. Rafter length is the sloped distance along the roof.
Does this calculate birdsmouth or plumb cuts?
No. This calculator estimates sloped length only. Cut layout, seat cuts, ridge details, and framing design are separate.
Can this size rafters structurally?
No. Rafter size, spacing, span, load, species, grade, and code requirements must be checked separately.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.