Roof Pitch Calculator
Turn rise and run, pitch, degrees, or percent slope into the other values.
How to measure
Rise and run
Rise is the vertical distance. Run is the horizontal distance. Use the same units for both, and measure to the same pair of points. On a common rafter, the run is half the building span, taken level. Do not run the tape along the slope and call that the run. A 6 inch rise over a 12 inch run is a 6/12 pitch. The same slope over a longer distance, such as 30 inches of rise in 60 inches of run, is still 6/12. A flat roof is a rise of 0. The run has to be greater than 0.
Pitch, degrees, and percent
Pitch on this page is the rise for 12 inches of run, written x/12. Degrees are the angle between the run and the slope. Percent is rise divided by run, times 100, so 6/12 is 50 percent. Enter any one of them. The page fills the other three, and it keeps the field you are typing as you left it. A pitch past 24/12, an angle of 90 degrees, or a percent that converts past 24/12 is out of range. Zero is a flat roof and is allowed.
Slope factor
The slope factor is the sloped length for one unit of horizontal run. It is the square root of 144 plus the pitch number squared, divided by 12. You do not measure it. Read it from the result and multiply the horizontal run by it when you want the rafter line length. The rafter calculator is the page that applies that factor to a span, an overhang, and the birdsmouth. This page does not size the rafter.
Formula
Pitch x/12 equals 12 times rise, divided by run. The angle in degrees is the arctangent of rise divided by run. Percent slope is that same ratio times 100. The slope factor is the square root of 144 plus x squared, divided by 12. Any one input is converted to x, and x must land from 0 through 24.
A flat roof is 0/12, 0 degrees, and 0 percent. A 12/12 pitch is 45 degrees and 100 percent. An angle of 90 degrees and a run of 0 are rejected because they are not a roof slope.
A 6/12 pitch is 26.57 degrees and 50 percent, with a slope factor of 1.118. A 4/12 pitch is 18.43 degrees, and a 12/12 pitch is 45 degrees.
Common mistakes
- Measuring the run along the rafter instead of on the level.
- Treating percent as the pitch number, so 50 is entered as 50/12.
- Using an angle of 90 degrees, or a run of 0, as if it were a pitch.
- Expecting this page to pick a rafter size or check a span table.
FAQ
What does a 6/12 pitch mean?
A 6/12 pitch rises 6 inches for every 12 inches of horizontal run. The angle is about 26.57 degrees, and the percent slope is 50, because rise divided by run is 0.5. The slope factor is the rafter length per inch of run, about 1.118, from the square root of 144 plus 36, divided by 12. Any one of those four inputs produces the other three. A 4/12 pitch is about 18.43 degrees, and a 12/12 pitch is 45 degrees.
How do I measure rise and run?
Rise is the vertical distance and run is the horizontal distance. On a roof, run is half the span for a common rafter, measured level, not along the slope. A 6 inch rise over a 12 inch run is 6/12. A longer run with the same slope, such as 30 inches of rise over 60 inches of run, is still 6/12. Measure to the same point on both sides, and do not follow the rafter with the tape.
What is the slope factor used for?
The slope factor is the hypotenuse of a triangle whose run is 12 and whose rise is the pitch number, divided by 12. Multiply the horizontal run by that factor to get the sloped length. A 6/12 factor of 1.118 means each inch of run needs about 1.118 inches of rafter. This page only reports the factor. The rafter length calculator applies it to a span, an overhang, and a birdsmouth.
Why is 24/12 the steepest pitch here?
This page accepts a flat 0/12 roof and pitches up through 24/12. Steeper than that is rejected, including a percent slope that converts past 24/12, such as 250 percent. An angle of 90 degrees is not a roof pitch, because the run would be zero. A run of zero is rejected for the same reason. If the slope you measured is past 24/12, check the rise and the run before you treat it as a rafter pitch.
Does this page size the rafters?
No. It converts pitch, angle, percent, and the slope factor. It does not choose a rafter size, a spacing, or a span table, and it does not check the IRC. Snow, wind, and the species of the lumber decide the member. Take the pitch into the rafter length calculator for the line length, the overhang, and the birdsmouth, then size the member from the span tables or a local design. The factor here is geometry only.
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By Lumber Calc Editorial Team · Verified against 2021/2024 IRC & AWC DCA 6 · Last updated Oct 24, 2026