Construction

Rafter Length Calculator

Q: How do I convert pitch to degrees?

Take the arctangent of rise divided by 12. For 8/12 that is about 33.7 degrees.

The rafter length calculator finds the length of a common rafter from the run and rise, and also converts between rise, run, pitch (like 6/12), angle and slope percentage, and works out the length of the common rafter, including the overhang. Use it to plan framing, order lumber and check a roof against shingle or metal minimums.

Pitch & angle
Rafter length
Overhang
Updated Nov 5, 2026

Pitch, slope and angle

Pitch states how many inches the roof rises for every 12 inches of horizontal run. A 6/12 roof rises 6 inches per foot. The same slope is an angle of about 26.6 degrees or a 50% grade.

Angle = arctan(rise ÷ run) and rafter length = √(rise² + run²)

Pitch	Angle	Typical use
3/12	14.0°	Low slope, needs suitable underlayment
4/12	18.4°	Common minimum for asphalt shingles
6/12	26.6°	Standard residential
9/12	36.9°	Steep, easier snow shedding
12/12	45.0°	Very steep, high labour cost

Example. A building 24 ft wide has a run of 12 ft to the ridge. At 6/12 the rise is 6 ft, the rafter length is 13.42 ft, and an 18 in horizontal overhang adds 1.68 ft along the slope, so the rafter is about 15.09 ft long.

The rafter length is the theoretical length measured along the slope. Ridge board thickness, birdsmouth cuts and local code change the actual cut, so confirm before cutting.

Questions people ask

What is the run?

The run is the horizontal distance from the outside wall to the centre of the building, which is half the span for a symmetrical gable roof.

What pitch do shingles need?

Standard asphalt shingles are generally installed on pitches of 4/12 or more; lower slopes need special underlayment or a different roofing type.

How do I convert pitch to degrees?

Take the arctangent of rise ÷ 12. For 8/12 that is arctan(0.667) = 33.7 degrees.