Roof pitch and slope
Roof Pitch Calculator
Enter measured dimensions to calculate slope and area factor.
Your quantity record
A personal result appears after you enter valid inputs and calculate.
Formula, diagram and sources
t = rise / horizontal run, or tan(angle × π / 180). Rise-in-12 = 12t; degrees = atan(t) × 180 / π; percent = 100t; area factor = √(1 + t²).
Equation version 2.0, checked October 14, 2026. Method and source records · Corrections
Last updated October 14, 2026 · Method · Corrections
Rise-in-12 states how far a roof rises for twelve level units. Degrees measure the angle above level. Percent slope is rise divided by run, multiplied by one hundred. Each describes the same measured slope in a different unit. Use level run, not full span or sloped length. Keep the unrounded ratio when converting area and check material needs in the exact system documents.
Sections7
The roof pitch calculator converts measured rise and level run, or a known angle, into rise-in-12, degrees and percent slope. It also gives the factor used to convert a matching footprint into sloped surface area. These calculations do not establish a covering's suitability or whether the roof is safe to walk on.
Rise-in-12 states how far a roof rises for twelve level units. Degrees measure the angle above level. Percent slope is rise divided by run, multiplied by one hundred. Each describes the same measured slope in a different unit. Use level run, not full span or sloped length. Keep the unrounded ratio when converting area and check material needs in the exact system documents.
Roof pitch calculator: choose your input mode
Choose rise and run when a plan gives a vertical rise and level distance. Use the same unit for both. Six inches of rise over twelve inches of run has the same ratio as half a foot over one foot. The calculator does not mix inches and feet within that pair.
Choose angle mode when the measured slope is already in degrees. Enter an angle of at least zero degrees and less than ninety degrees. A vertical face has no finite rise-to-level-run ratio, so this roof-area conversion cannot handle it.
Unknown slope stays unknown. A roof style or a photograph does not supply a measured ratio. Get the slope from an existing plan or report. Read how to calculate roof pitch for the unit and source checks.
Roof pitch calculator equations
Let t be rise divided by level run. Rise-in-12 is 12 × t. Angle is atan(t) × 180 / π. Percent slope is 100 × t. The area factor is √(1 + t²).
For angle input, t = tan(angle × π / 180). The calculator uses that ratio for each output, retaining unrounded values and rounding only the display. The roof pitch chart provides common integer pitches as a printable reference.
An area factor belongs to level plan area. A report that already gives sloped roof surface needs no second factor. The roofing calculator separates those modes to keep pitch from being counted twice.
Worked slope conversion
Assume a measured rise of six inches and level run of twelve inches. The ratio is one-half. The output is 6:12, about 26.565 degrees, fifty percent slope and an area factor of about 1.118034. These numbers are derived from a stated example, not a typical roof assumption.
At 12:12, the angle is forty-five degrees and percent slope is one hundred. A one-hundred-percent slope is not vertical. It means rise equals level run. At zero rise, angle and percent are zero and the area factor is one.
| Output | Unit | Use |
|---|---|---|
| Rise-in-12 | Ratio with run of 12 | Common roof slope notation |
| Angle | Degrees above level | Measured angular input |
| Percent slope | Rise / run × 100 | Another way to state the ratio |
| Area factor | Unitless | Convert matching projected area |
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Slope and material fit are separate checks
A numeric conversion is not approval for shingles or metal. GAF's slope guidance places conditions on its shingle systems. Metal needs differ by profile and seam type. Use minimum pitch for metal and minimum pitch for shingles for those source checks.
The calculator does not assess drainage, snow loads or the state of the deck. A level mathematical input is accepted for conversion; it is not a claim that a level roof can use a particular covering. Read low pitch roofing materials when the assembly choice is unsettled.
Sources, print record and limits
The equations follow basic trigonometry and were checked independently on October 14, 2026. GAF's roofing-square guide explains the roof-surface unit. The printed result includes the inputs and method, keeping the source ratio visible alongside the rounded outputs.
Frequently asked questions
What is roof pitch?
In common roofing use, pitch often means rise per twelve level units. This site labels that output rise-in-12. Some older references use rise divided by full span for pitch, which is a different ratio. Read the source's definition before entering a number; do not silently treat span as level run.
Is 6:12 the same as six degrees?
No. Six rise units over twelve run units gives an angle of about 26.565 degrees. The ratio and angle use other units. Choose the input mode that matches the measured record. If the plan states degrees, use angle mode instead of copying that number into the rise-in-12 field.
Is a one-hundred-percent slope vertical?
No. It means rise equals level run, which gives a forty-five-degree angle. Percent slope grows without a finite limit as the angle approaches vertical. The angle mode therefore needs less than ninety degrees. This mathematical distinction does not find material suitability or safe roof access.
Can the factor be applied to measured roof surface?
No second factor belongs on a surface that already includes slope. The factor converts level plan area to sloped surface. Applying it again enlarges the same roof twice. Keep the report's area basis with the count result and use known-surface mode in the roofing calculator when that is what the source gives.
Verdict: preserve the slope unit
Use the roof pitch calculator to convert a known slope, then keep its measured basis. Open the roof pitch chart for common values or check the exact covering's slope limits. The method page keeps the equations and correction route.