Roof Pitch Calculator
A 6-inch rise over a 12-inch run equals a 6/12 roof pitch, a 50 percent slope, and about a 26.6 degree roof angle. Use this roof pitch calculator to turn rise and run into the three roof slope formats builders use most. Enter the vertical rise and horizontal run in inches to get pitch per 12, slope percent, and the roof angle.
Quick answer
Roof pitch is usually written as rise per 12 inches of horizontal run.
What this tells you
- •Roof pitch is usually written as rise per 12 inches of horizontal run.
- •Slope percent is rise divided by run, multiplied by 100.
- •Roof angle is the angle from horizontal, found with the arctangent of rise divided by run.
- •Rise and run must use the same unit even though the form labels both values in inches.
- •The result converts geometry only. It does not approve a roof design, material, or work method.
How to Use
- 1Measure the vertical rise in inches.
- 2Measure the horizontal run in inches. For a simple gable roof, this is usually half the building span, not the full width.
- 3Enter both values and click Calculate.
- 4Read the result as pitch per 12, slope percent, and roof angle in degrees.
How It Works
Formula
Pitch per 12 = (Rise / Run) x 12
Slope Percent = (Rise / Run) x 100
Roof Angle = atan(Rise / Run) x 180 / piAll three outputs come from the same rise-to-run ratio. A roof with 6 inches of rise and 12 inches of run has a ratio of 0.5. Multiply by 12 to get a 6/12 pitch, multiply by 100 to get a 50 percent slope, and take the arctangent to get an angle of about 26.6 degrees.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Common 4/12 roof pitch
Divide 4 by 12 to get 0.3333. That ratio becomes 4/12 when scaled to 12 inches of run, 33.3 percent as a slope, and about 18.4 degrees as a roof angle.
Common 6/12 roof pitch
Divide 6 by 12 to get 0.5. Multiply by 12 for a 6/12 pitch and by 100 for a 50 percent slope. The roof angle is about 26.6 degrees.
Steep 12/12 roof pitch
When rise and run are equal, the ratio is 1. That gives a 12/12 pitch, a 100 percent slope, and a 45 degree roof angle.
Common Roof Pitch Reference
Quick conversions for standard residential roof pitches.
| Pitch | Slope Percent | Roof Angle |
|---|---|---|
| 3/12 | 25.0% | 14.0° |
| 4/12 | 33.3% | 18.4° |
| 6/12 | 50.0% | 26.6° |
| 8/12 | 66.7% | 33.7° |
| 9/12 | 75.0% | 36.9° |
| 12/12 | 100.0% | 45.0° |
Run means horizontal distance. Do not use rafter length for this table.
What does roof pitch mean?
Roof pitch compares vertical rise to horizontal run. On a straight gable-style roof, rise is the height from the top of the wall line to the ridge, and run is the horizontal distance from the outside wall to the ridge centerline.
Builders often switch between pitch, slope percent, and degrees depending on the task. Pitch is common on plans, slope percent is common in drainage and estimating discussions, and degrees help with saw settings and angle checks.
This calculator only converts the slope geometry. Use a separate roofing material calculator when you need squares, bundles, or cost.
You can measure pitch from accessible framing with a level or framing square. Establish a known horizontal run, then measure the vertical rise over that distance. A 12-inch level run makes the rise reading the familiar pitch-per-12 number. Keep the reference truly horizontal and avoid measuring along the sloped rafter.
Plan drawings may give total span, ridge height, or a pitch mark. For a centered simple gable, one roof run is usually half the wall-to-wall span. Overhang does not change the main roof pitch, but it adds horizontal run and rafter length beyond the wall. An offset ridge, unequal slopes, or additions require each roof plane to be handled separately.
Pitch affects material instructions and detailing. Roofing products can have minimum slopes, special underlayment rules, fastening patterns, or restrictions at low slopes. Snow, wind, drainage, ventilation, flashing, and structural loads are separate design questions. A matching angle does not show that the rafters, trusses, sheathing, or connections are adequate.
Measuring an existing roof from the ground, attic, or plans is safer than climbing onto it. Roof work involves fall hazards, fragile surfaces, electrical hazards, weather, and unstable ladders. Use trained professionals and required fall protection for any task that cannot be completed from a safe location.
The output rounds pitch to two decimals and slope percent and angle to one decimal. That is useful for a quick check but may be too coarse for fabrication or layout over a long distance. Use the original measured ratio and appropriate construction drawings when accumulated error matters.
Common mistakes
- Using the full building width as the run on a gable roof instead of the horizontal distance from wall to ridge.
- Mixing units, such as inches for rise and feet for run.
- Measuring along the rafter instead of measuring horizontal run.
- Expecting this calculator to estimate shingles, underlayment, or other materials.
- Including the overhang when the needed run is only from the wall line to the ridge.
- Assuming a converted angle proves that a roofing product is allowed at that slope.
- Climbing onto a roof for a measurement that could be taken from plans, framing, or a safer location.
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