Roof Pitch Calculator
Pitch, angle, percent, and the multiplier that turns flat measurements into real roof area — the geometry under every roofing number.
Geometry only. The multiplier converts flat (plan) area to sloped roof area — the Roofing Calculator turns that into squares and bundles.
How the math works
Pitch is rise over run, stated per 12. Angle = arctan(rise ÷ run). The roof-area multiplier is √(1 + (rise ÷ run)²) — multiply a flat (plan) measurement by it and you have the sloped length or area. Percent slope is just rise ÷ run × 100, which is why a “100% slope” is a 12:12 roof at 45°.
A worked example
An 8 in rise over a 12 in run is an 8:12 pitch: 33.69°, 66.67% slope, a multiplier of 1.20185. Over 20 ft of horizontal run, the sloped length is 24.04 ft — the extra four feet is what flat measurements forget.
Questions from the field
How do I find my roof pitch? Measure 12 inches level from the roof surface or a rafter, then the vertical distance down to the surface at that point. That vertical number is your pitch numerator. Attic rafters are the safest place to measure.
What does the multiplier do? It converts flat area to roof area. A 1,000 sq ft footprint at 8:12 is 1,202 sq ft of roof — the Roofing Calculator uses it to get squares and bundles.
Is pitch the same as slope percent? No — an 8:12 pitch is a 66.67% slope and a 33.69° angle. Three languages for one roof; this page translates between them.
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