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Roof Rafter Length Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Formula based on Pythagorean theorem with standard pitch notation. Last verified 15 June 2026.

Calculating rafter length is one of the most common framing tasks in residential construction. The rafter is the hypotenuse of a right triangle formed by the horizontal run and the vertical rise. Given a roof pitch expressed as X-in-12 (X inches of rise per 12 inches of run), the rise = Run x (Pitch / 12), and rafter length = sqrt(Run squared + Rise squared). This calculator also gives the roof angle in degrees and the pitch multiplier, which is a useful factor for computing roof area. Enter the horizontal run (half the building span for a simple gable) and the pitch.

Half the building span in feet (ridge to wall plate)
Enter the rise number (e.g. 6 for a 6/12 pitch)
Horizontal overhang beyond the wall plate in feet
0.00
0.00
0.00
0.00
0.00

Rafter length formula

Rise = Run x (Pitch / 12)
Rafter (to ridge) = sqrt(Run^2 + Rise^2)
Tail = sqrt(Overhang^2 + (Overhang * Pitch/12)^2)
Total rafter = Rafter to ridge + Tail

The pitch multiplier (rafter factor) equals sqrt(1 + (pitch/12) squared). Multiply the horizontal run by the pitch multiplier to get rafter length without computing rise separately. The roof angle in degrees is arctan(pitch / 12) converted from radians.

Common roof pitches

  • 2/12 to 3/12: low-slope roof, requires special waterproofing measures.
  • 4/12 to 6/12: moderate pitch, most common in residential construction.
  • 7/12 to 9/12: steep pitch, used for aesthetic and snow-shedding purposes.
  • 10/12 to 12/12: very steep, requires additional safety measures during construction.
  • Above 12/12: extremely steep, uncommon in standard residential design.

Roof rafter calculator: frequently asked questions

How do I calculate rafter length from pitch and run?

Rafter length = sqrt(run squared + rise squared). First calculate rise from pitch: Rise = Run x (pitch / 12). For a 6/12 pitch with a 12-foot run, rise = 12 x 6/12 = 6 feet, rafter = sqrt(144 + 36) = sqrt(180) = 13.42 feet.

What is roof pitch?

Roof pitch is the ratio of vertical rise to horizontal run, expressed as X-in-12. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run. Common residential pitches range from 4/12 to 12/12.

What is the difference between span and run?

Span is the total horizontal width of the building, measured from outside wall to outside wall. Run is half the span for a simple gable roof (the horizontal distance from the exterior wall to the ridge). Enter the run (half-span) in this calculator.

Do I need to add extra length for the rafter tail (overhang)?

Yes. The rafter length from this calculator is to the ridge. Add the overhang length (eave extension) separately. A typical residential overhang is 12 to 24 inches, calculated as: overhang = sqrt(eave_run squared + eave_rise squared) using the same pitch ratio.

What is the minimum roof pitch for shingles?

Asphalt shingles require a minimum 2/12 pitch with ice-and-water shield underlayment, and a minimum 4/12 for standard installation per most shingle manufacturer warranties and local codes.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.