Roof Pitch Calculator

Calculate roof pitch, slope percentage, angle and slope factor from a measured rise and horizontal run, each with explicit units.

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How to Use the Roof Pitch Calculator

Enter vertical rise and horizontal run, selecting inches or feet independently. Run is the level horizontal distance, not the sloping rafter length. Optionally enter the full horizontal span of a symmetric gable roof to estimate its rise; the calculator uses half that span as the run. Reset restores six inches rise over twelve inches run.

Formula and Explanation

Normalize rise and run to feet (inches / 12)
Ratio = vertical rise / horizontal run
Pitch per 12 = ratio × 12
Slope percentage = ratio × 100
Angle in degrees = atan(ratio) × 180 / π
Slope factor = sqrt(1 + ratio²)
Symmetric gable rise = full horizontal span / 2 × ratio

Pitch written X:12 expresses vertical rise for twelve equal horizontal units. Slope percentage expresses rise for one hundred horizontal units; neither is the angle in degrees. Slope factor relates a sloping length to its horizontal projection for the measured plane. This tool labels the common roof-pitch convention explicitly; terminology can differ in older references.

A zero run is invalid; zero rise is a valid flat arithmetic case. The optional rise assumes a symmetric gable with the ridge centered over the full entered span and excludes overhang geometry. A shed roof’s full run must not be treated as a gable span. The tool does not check structural adequacy, roof access, local code or material suitability. Use measurements from plans or an appropriate measurement method; no roof climbing is required by the calculator.

Worked Example

Six inches of rise over twelve inches of horizontal run gives ratio 0.5, pitch 6:12, slope 50%, angle about 26.565051° and slope factor about 1.118034. For a symmetric roof with a 24-foot full span, the half-span run is 12 feet and the estimated ridge rise is 6 feet. Six inches over one foot gives the same pitch.

Frequently Asked Questions

Can rise and run use different units?

Yes. Each selector is converted to feet independently before calculating the ratio. Six inches of rise and one foot of run equal 6:12.

Why is 6:12 a 50% slope but not a 50-degree angle?

Slope is 6 divided by 12, or 0.5. The angle is arctan(0.5), approximately 26.565 degrees.

What does slope factor mean?

Multiply horizontal projected length by the factor to obtain sloping length on that roof plane. At 6:12 the factor is about 1.118, before any overhang or waste adjustment.

How is optional roof rise calculated from span?

For the stated symmetric-gable assumption, half the full span is the horizontal run. Multiply that half-span by the rise/run ratio. Leave span blank if that assumption does not fit.