Home Improvement
Roof Pitch Calculator
Enter rise and run to see pitch as x:12, degrees and grade percent.
What is roof pitch?
Roof pitch describes how steep a slope is, which drives material choice, drainage behaviour and whether a covering is allowed by the manufacturer. Measure rise (vertical) and run (horizontal) in the same units over one consistent section of the plane; the calculator scales that ratio to the trade “x : 12” notation, converts the angle with arctan(rise ÷ run), and reports grade percent as rise ÷ run × 100. Keep rise and run matched — mixing centimetres with inches breaks the ratio. Always confirm tile, slate or membrane minimum pitches on the product sheet before ordering. For covering quantity after you know the geometry, see the roof tile calculator.
Worked example
Rise 20 and run 30 (same units) → 8 : 12 pitch, about 33.7° and 66.7% grade (slope 0.667).
Limits and assumptions
Geometry only — not structural design, snow or wind load, rafter sizing or building-control approval.
Frequently asked questions
What are rise and run?
Rise is the vertical height gained; run is the horizontal depth of the slope segment you measured. Use the same length unit for both.
Why express pitch as x : 12?
Many UK and US trade specs quote inches of rise per 12 inches of run so coverings and flashings share a common language.
Can I measure in millimetres?
Yes. The ratio and degrees are unitless as long as rise and run use the same unit.
Does this work for a hip or valley?
Measure each plane’s rise and run separately; intersecting hips and valleys need their own pitch checks.