Roof pitch calculator

Measure how much the roof rises over a horizontal run, then divide rise by run. Expressed per 12 units of run it becomes the X-in-12 pitch roofers use; take the arctangent of rise ÷ run to get degrees. Enter the two numbers above and the calculator does all three at once.

A 4:12 pitch rises 4 units for every 12 units of horizontal run, which works out to about 18.4° (arctan of 4 ÷ 12), or a 33.3% slope. It sits in the most common range for asphalt-shingle roofs.

Take the tangent of the angle and multiply by 12. For a 30° roof, tan(30°) × 12 ≈ 6.9, so the pitch is roughly a 7:12. The degrees-to-pitch converter above does this for any angle.

Most residential roofs fall between 4:12 and 9:12 (about 18° to 37°). Shallower than 2:12 counts as a low-slope roof and usually needs membrane roofing instead of shingles; steeper than 9:12 is considered steep-slope and harder to walk.

Up to about 6:12 (26.6°) most people can walk a dry roof with care. Between 7:12 and 9:12 it gets difficult and needs roof jacks or staging; above 9:12 assume it is not walkable without equipment. Always err on the side of staying off the roof — you can measure the pitch from the attic or the ground instead.

Hold a phone with an angle app flat against a rafter in the attic — rafters share the roof’s exact slope — or take a side-on photo of the gable and measure the angle in the photo with a camera protractor. Both give you the pitch without a ladder.