Start with the right measurement
Roof pitch is a ratio, not a measurement along the shingles. This calculator helps you translate a level run and vertical rise into the notation used when comparing roof plans, cutting lists, and material estimates.
How to use this calculator
- 01Rise: Vertical change over the measured run.
- 02Run: Level horizontal distance; it must be greater than zero.
The pitch notation is easiest to compare with roofing tables; angle and slope percentage are alternate views of the same geometry. The multiplier is the sloped-length factor used for surface estimates.
Units: Inches, feet, centimetres, and metres all work. Rise and run only need to use the same unit; the ratio cancels the unit.
How the calculation works
Pitch = rise ÷ run. Angle = arctan(rise ÷ run). The X:12 result multiplies that ratio by 12.A 6-inch rise over a 12-inch run is a 6:12 pitch, a 26.57° angle, and a 50% slope.
Worked examples
Low-slope porch roof
- Measure a 12 in horizontal run.
- Measure 3 in of vertical rise.
- Divide 3 by 12: 0.25, then multiply by 12 for notation.
Result: The roof is 3:12, about 14.04°, and 25% slope.
Steeper gable
- Use a 10 in run and 8 in rise with the same units.
- Compute 8 ÷ 10 = 0.8.
- Convert the ratio to an X:12 pitch: 0.8 × 12.
Result: The result is 9.6:12, about 38.66°, and 80% slope.
How to measure
- 01Measure rise and run using the same unit.
- 02Keep the level run horizontal—not along the roof surface.
Common mistakes to avoid
- Mixing inches and feet
- Measuring the sloped length as run
- Rounding before the final step
Safety note
Do not climb or measure a roof without appropriate fall protection and a safe access plan.
Limits of this estimate
This geometry does not evaluate roof structure, loading, drainage, flashing, or local rules.
Frequently asked questions
Is 6:12 the same as 50%?
Yes. Six divided by twelve is 0.5, or a 50% slope.
Can rise and run be in centimetres?
Yes. The ratio is unitless as long as both use the same unit.