Roof Pitch Calculator
Convert roof slope between four formats and see a labelled triangle. Calculate an optional sloping area and geometric length from your project measurements.
Convert roof pitch, angle and slope
Choose the measurement you know. In rise-and-run mode, both measurements must use the same unit: 6 inches of rise over 12 inches of horizontal run is a 6:12 slope. In the other modes, only the corresponding pitch, angle or percentage field is used. Project run is a separate full-size horizontal measurement used for the geometric slope length.
Formula and worked example
Let s = rise ÷ run. Pitch = 12s, percentage = 100s, angle = arctan(s), and area factor = √(1 + s²). A 6:12 roof has s = 0.5, angle approximately 26.565 degrees and factor 1.118. A 10-foot horizontal run gives 11.180 feet of geometric slope length. A 1,000-square-foot horizontal projection gives approximately 1,118.034 square feet of sloping surface.
Is run the same as the building width?
No. For a symmetrical gable, one slope's run is usually half the span between the supports, before any horizontal overhang adjustment. A shed roof uses its full horizontal run for that slope. Measure the run relevant to the roof plane, not the distance along the sloped roof.
Does this give a rafter cutting length?
No. It gives the hypotenuse between the two points represented by the run and rise. Ridge thickness, seat cuts, overhangs, allowances and structural design are not included. The diagram illustrates the slope and is not a construction drawing.
How do I use the roof area factor?
Multiply the horizontal projected area of a plane by its slope factor. Calculate planes with different slopes separately. Entering the footprint of both equal-pitch gable halves already includes both sides; do not double it again. The result does not include ordering waste.
What does zero pitch mean?
Zero returns a mathematically flat surface. This is not a recommendation for drainage or any roofing material. Minimum installation slopes and safe access depend on the roof system and local requirements. Use measurements taken safely; the tool cannot assess roof suitability.
Reference: Rise/run convention and roof-pitch explanation. Calculations use standard right-triangle trigonometry.
Next: use the pitch and area in our Roof Square Footage Calculator for material allowance. Also see the Stair Stringer Calculator.
Reviewed 4 November 2026. Free, no signup. Inputs stay in your browser. Quantities are estimates; check measurements and product instructions before purchasing.