BuildProjectCalc
Roofing guide

How to Calculate Roof Pitch

Use rise and run to calculate a roof pitch ratio, slope percentage, and angle in degrees without making structural or code assumptions.

The short answer

Roof pitch compares vertical rise with horizontal run. Divide rise by run for the slope, multiply by 100 for slope percentage, and take the arctangent of rise ÷ run for the angle. A 6:12 pitch rises 6 units for every 12 horizontal units.

Use the Roof Pitch Calculator

Four ways the same roof slope is described

Rise
The vertical change over the measured horizontal run.
Run
The horizontal distance, not the sloped rafter or roof-surface distance.
Pitch ratio
In common US notation, rise is normalized to a 12-unit run, such as 6:12.
Percentage and degrees
Slope percentage is rise ÷ run × 100. The angle is arctangent(rise ÷ run).

Roof pitch conversions

Pitch per 12 = (rise ÷ run) × 12
Slope percentage = (rise ÷ run) × 100
Angle = arctan(rise ÷ run)

Rise and run may use inches, centimeters, or another common unit as long as both measurements use the same unit.

Worked example: a 6:12 pitch

Suppose the roof rises 6 inches during 12 inches of horizontal run.

  1. Rise-to-run ratio: 6 ÷ 12 = 0.5
  2. Pitch ratio: 6:12
  3. Slope percentage: 0.5 × 100 = 50%
  4. Angle: arctan(0.5) = 26.565°

Result: A 6:12 pitch corresponds to a 50% slope and an angle of about 26.6 degrees. These are different expressions of the same geometry.

Measure without confusing run and roof surface

  1. Choose two points on the same slope

    The measurements must describe one consistent roof plane.

  2. Measure horizontal run

    Keep the run level; do not substitute the diagonal roof length.

  3. Measure vertical rise

    Measure the vertical change between the endpoints of that horizontal run.

  4. Use matching units

    Convert one measurement if necessary so rise and run share the same unit.

Limits of a pitch calculation

Pitch conversion describes geometry only. It does not determine structural capacity, roofing-system compatibility, drainage performance, safe access, or code compliance.

When roof dimensions are difficult or unsafe to obtain, use verified drawings or measurements collected by an appropriate professional rather than working from an exposed roof surface.

Roof pitch questions

Is a 50% slope the same as 50 degrees?

No. A 50% slope means rise is half the run; its angle is about 26.6 degrees.

Does a 6:12 pitch require inches?

No. The ratio is unitless. Six centimeters of rise over 12 centimeters of run has the same geometric slope.