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Free roof pitch calculator

Roof Pitch Calculator

Turn rise and run, or an angle, into pitch, degrees and rafter factor.

Measure 12" level along the rafter, then straight up to it: that is the rise per 12.

Rafter length for a span

How to use the roof pitch calculator

  1. Measure the vertical rise over a level run — 12" of run makes it simple.
  2. Or switch to Angle if you know the roof angle in degrees.
  3. Open Rafter length for a span to get the slope length over any horizontal run.

Roof pitch formulas

Pitch = rise ÷ run × 12 (written x/12)

Angle = arctan(pitch ÷ 12) · Slope % = pitch ÷ 12 × 100

Rafter factor = √(1 + (pitch ÷ 12)²)

Every pitch from 1/12 to 24/12, with its angle, rafter factor and suitable roofing, is in the roof pitch chart. To cut the rafters themselves, use the rafter calculator.

Frequently asked questions

How do I calculate roof pitch?

Measure the rise over a known horizontal run, then scale it to 12: pitch = rise ÷ run × 12. A roof that rises 30" over 60" of run is 30 ÷ 60 × 12 = 6, a 6/12 pitch.

How do I measure roof pitch without getting on the roof?

From the attic, hold a level against the underside of a rafter, mark 12" along the level, and measure straight up from that mark to the rafter. Or measure the gable end from the ground: the height of the peak above the wall top over half the building width.

How do I convert roof pitch to degrees?

Angle = arctan(rise ÷ 12). A 4/12 roof is 18.4°, 6/12 is 26.6°, 8/12 is 33.7°, 12/12 is 45°.

What is the most common roof pitch?

On US houses, between 4/12 and 9/12 — steep enough for asphalt shingles with standard underlayment, and still walkable. 6/12 is a typical middle value.

What is the minimum pitch for shingles?

The IRC allows asphalt shingles down to 2/12, but slopes from 2/12 to 4/12 need two layers of underlayment. Below 2/12 you need a low-slope membrane or metal rated for it.

What is the rafter factor?

The length of rafter per foot of horizontal run: √(1 + (pitch ÷ 12)²). Multiply the run by it to get the rafter length along the slope — 1.118 for a 6/12 roof.