Roof Pitch Calculator (Rise and Run to Pitch)

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Measure a rise and a run anywhere on the roof or rafter. A level held out 12 in with a tape measured straight down to the roof is the classic way. Enter both numbers and the calculator gives the pitch, the angle and the slope factor, and says whether asphalt shingles are allowed at that slope.

Roof pitch calculator

How it works

  • Pitch = rise ÷ run × 12, written as “x in 12”
  • Angle = arctan(rise ÷ run)
  • Slope % = rise ÷ run × 100
  • Slope factor = √(1 + (rise ÷ run)²), the length of roof per unit of level run

The IRC does not allow asphalt shingles below 2 in 12 and requires double underlayment from 2 in 12 up to 4 in 12 (R905.2.2).

Worked example

An 18 in rise over 36 in of level run:

  • Rise measured: 18 in
  • Run measured (level): 36 in

The calculator gives:

  • Roof pitch: 6 in 12
  • Angle: 26.57°
  • Slope as a percentage: 50.0%
  • Slope factor (rafter length per foot of run): 1.1180
  • Nearest whole pitch: 6/12

18 ÷ 36 × 12 = 6, so the roof is 6 in 12. The angle is arctan(0.5) = 26.57°.

Notes and limits

  • Measure run level and rise plumb. A sagging rafter or bent level gives a wrong answer, so take two or three readings.
  • Pitch and slope are often used interchangeably. Strictly, “pitch” once meant rise over the whole span, so a 6 in 12 slope was a 1/4 pitch. Most people and the IRC now use rise per 12 in of run.

Frequently asked questions

How do I find my roof pitch?

Hold a level horizontally against the roof or a rafter, mark 12 in along it, and measure straight down (or up) to the roof. That measurement is the pitch in 12.

What is a common roof pitch?

Pitches from 4/12 to 9/12 are common on houses. Low-slope roofs are under 3 or 4 in 12, and steep roofs are over 9/12.

What pitch is too low for shingles?

Under 2 in 12 the IRC does not allow asphalt shingles. From 2/12 to 4/12 they need double underlayment.

Sources

  1. International Code Council, 2021 International Residential Code, Section R905.2.2 slope limits for asphalt shingles

Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.

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