FIELD MEASUREMENT

Figuring Roof Pitch: Measure Rise and Run

Hold a level horizontally, mark a 12-inch run, then measure straight down to the roof surface at that mark. The vertical measurement is the first number in an x:12 roof pitch.

Calculate a roof pitch

A level held horizontally above a roof surface, with a horizontal baseline and a vertical tape measurement showing roof rise.
Pitch uses a horizontal run and a vertical rise—not the sloped rafter distance.

How do I work out the pitch of my roof?

Use a level as the horizontal reference. Over a 12-inch horizontal run, measure the vertical gap between the level and the roof surface. A 4-inch gap means 4:12; a 6-inch gap means 6:12. If your level reference is longer or shorter than 12 inches, convert it with the formula below.

The 12-inch level method

  1. Choose one straight section of the roof plane and establish a horizontal reference with a level.
  2. From the point touching the roof, mark 12 inches along that horizontal reference.
  3. At the 12-inch mark, measure vertically to the roof surface.
  4. Write the vertical measurement as the rise in the format rise:12.

Keep the baseline level. Measuring along the roof surface gives a diagonal distance, which is not the run used for pitch.

When the horizontal run is not 12 inches

Any horizontal run works if you scale the rise to a 12-inch run:

pitch = (rise ÷ run) × 12Write the result as x:12.
angle = atan(rise ÷ run)Convert the ratio to degrees.

For example, 9 inches of rise over 24 inches of horizontal run gives (9 ÷ 24) × 12 = 4.5, so the pitch is 4.5:12. The calculation uses matching units for both measurements.

Check the measurement before using it

Repeat the measurement on the same roof plane if the level shifts or the surface is uneven. This page explains roof geometry only. It does not determine material suitability, drainage design, structural sizing, or local requirements.

Figuring roof pitch FAQ

Can I use a 24-inch level?

Yes. Measure the vertical rise over its horizontal run, then multiply rise divided by run by 12. For instance, a 9-inch rise over 24 inches becomes 4.5:12.

Why must the run be horizontal?

Roof pitch compares vertical rise with horizontal run. A rafter or tape laid on the roof follows the diagonal instead, so it is a different measurement.

Is a pitch ratio the same as a roof angle?

They describe the same slope in different forms. The ratio is rise per 12 units of horizontal run; the angle comes from the inverse tangent of rise divided by run.

Editorial question addressed: “How do i work out the pitch of my roof” (r/DIY; source date was not provided in the site roster). The answer above is an original geometry explanation.