Quick answer
Rafter Length Calculator: direct answer
Rafter Length Calculator helps you estimate rafter length from rise and run while also checking roof pitch, angle, and pitch multiplier. It is best for estimating the sloped length from roof rise and run and returns pitch ratio, roof angle in degrees, slope percentage for material planning.
Use this calculator when you know rise and run, pitch ratio such as 6:12, angle in degrees when converting from angle. The estimate uses this rule: rafter length = square root of rise squared plus run squared.
Inputs
- Rise and run
- Pitch ratio such as 6:12
- Angle in degrees when converting from angle
- Consistent measurement units
Outputs
- Pitch ratio
- Roof angle in degrees
- Slope percentage
- Pitch multiplier
- Rafter length
Formula
How this estimate works
rafter length = square root of rise squared plus run squared
In plain terms, compare rise to horizontal run, convert that ratio into pitch, degrees, slope percent, and multiplier, then use the result for roof planning.
A 6 in rise and 12 in run gives a unit rafter length of about 13.42 in.
Use cases
When to use this calculator
Estimate geometric rafter length before accounting for cuts, ridge details, and overhangs.
Check whether rise and run produce the pitch expected on the plan.
Use the calculated length as an early planning number before selecting actual lumber lengths.
Worked example
Find rafter length from rise and run
Rafter length is the sloped side of a right triangle, so the calculator uses rise and horizontal run to estimate the geometric length.
- Enter the vertical rise and horizontal run using consistent units.
- Review rafter length before adding overhang or framing details.
- Use pitch and angle results to cross-check the roof slope.
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.
Measurement tips for a better estimate
- Run is horizontal distance, not the sloped distance along the roof.
- Add overhang, ridge, and cut details separately from the geometric result.
- Keep rise and run in the same unit to avoid a distorted length.
Common estimating mistakes
- Using building width as run without dividing by the correct roof geometry.
- Treating calculated rafter length as final cut length.
- Mixing inches and feet in rise and run entries.
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.