Roof pitch and slope
How To Calculate Roof Pitch
Last updated October 14, 2026 · Method · Corrections
Enter measured dimensions to calculate roofing quantities.
Use level run for a slope ratio. Full span includes both sides of an equal-sided gable, while slope length runs along the roof. Neither can replace run without a stated conversion. For rise-in-12, multiply rise divided by run by twelve. Degrees and percent use other equations. A correct ratio helps with area. But exact manufacturer documents still control the roof covering.
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How to calculate roof pitch is a rise-and-run task. Divide vertical rise by level run, using the same unit, then multiply by twelve for rise-in-12. If the starting record gives degrees, use the tangent of the angle instead. This guide checks existing measured data; it does not need roof access.
By the end of this guide, you'll know how to check rise and run, convert slope units, and carry the unrounded ratio into an area worksheet.
How to calculate roof pitch from rise and run
Start with a measured plan or roof report. Identify the vertical rise and level run for the same plane. Check their units. A six-inch rise divided by a one-foot run must first use one unit: six inches over twelve inches, or half a foot over one foot.
For a simple equal-sided gable, level run is half the full eave span. The rise is measured from the level eave line to the ridge. Do not use a sloped rafter length as the denominator. That gives a different triangle ratio.
Enter the pair in the roof pitch calculator. It shows rise-in-12, degrees, percent and area factor. Keep the input source with the printed record. If the plan does not state the needed sizes, leave slope unknown instead of assigning a common value.
How to calculate roof pitch from an angle
For a known angle θ above level, the slope ratio is tan(θ × π / 180). Multiply by twelve to get rise-in-12. Multiply by one hundred to get percent slope. The angle must be in degrees for this formula as written.
A forty-five-degree angle gives equal rise and run, or 12:12. It is a one-hundred-percent slope. A six-degree angle gives a much shallower ratio than 6:12. Copying the angular number into a rise field can therefore alter the area and material check.
Calculate the roof area factor
For ratio t, the area factor is √(1 + t²). An assumed 6:12 slope has t equal to one-half and factor about 1.118034. Multiply the matching level plan area by that factor. Do not apply it to actual sloped surface.
Use a separate ratio for each plane when slopes differ. A main roof and lower porch can have different pitches. One average ratio is not a replacement for their measured areas. The roofing calculator gives a separate-plane workspace.
Common mistakes to avoid
- Mixing length units: convert rise and run to the same unit first.
- Using full span as run: use the appropriate half-span only for a confirmed symmetrical gable.
- Using slope length as run: distinguish the sloped side from its horizontal projection.
- Confusing degrees with rise-in-12: select the matching conversion mode.
- Adding slope twice: leave already measured surface unchanged.
The pitch chart can check common ratios. Its rounded factor is for reference, not a replacement for the exact input. A result close to a listed row should keep its actual measured ratio instead of being rounded to a more familiar pitch.
Sources and covering limits
GAF's slope guide defines slope using rise over level run. The conversion equations were derived and checked on October 14, 2026. Check the selected roofing system separately against minimum shingle pitch or metal slope limits.
Frequently asked questions
What is roof pitch and why work it out?
The slope ratio helps convert plan roof area and check a covering's stated pitch range. It does not inspect the roof or approve a system. This site uses rise-in-12 as its common pitch notation. Keep the source measured units and exact source so the conversion can be checked without assuming a typical slope.
Can I calculate pitch from a photograph?
A photograph is not a reliable measured slope record by itself. Camera angle and perspective can alter the apparent roof angle. Use a measured plan or measured report instead. This guide does not ask you to climb onto the roof to fill a missing input; unknown slope should stay unresolved until a valid record exists.
Do different roof planes need separate pitches?
Yes when their slopes differ. A common ratio works only for planes that share that measured slope. Give each other plane its own plan area and factor, then add the surfaces. If a report already measures the sloped planes directly, use those areas without applying another pitch factor to the total.
Does the calculated pitch choose the material?
No. Material suitability also depends on the exact system, its current instructions and local rules. A metal seam type or shingle assembly can have conditions beyond a minimum ratio. Use the material-specific guides with the exact product documents. A mathematical conversion does not show drainage performance or safe access.
Verdict: check the triangle before the conversion
Use how to calculate roof pitch to distinguish rise, run and slope length. Keep units consistent and keep the unrounded ratio. Return to the roof pitch calculator for the outputs and Method and Sources for the kept equation record.