Roof Pitch Calculator
Enter the rise and run. Get the pitch, angle and rafter length.
100% in your browser - no upload, no sign-up.
Pitch (x-in-12)
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Angle: -
Slope factor
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Grade
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Rafter length
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✅ Standard slope (4:12 or steeper) - suitable for asphalt shingles and most steep-slope roofing.
⚠️ Low slope (2:12 to under 4:12) - shingles are allowed only with a double underlayment; check the manufacturer's spec.
⛔ Too low for shingles (under 2:12) - use a low-slope membrane such as EPDM, TPO or built-up roofing.
💡 These are planning numbers. Minimum slopes and underlayment rules vary by roofing material and local code - confirm the manufacturer's spec and your local building code before ordering, and add rafter length for the overhang and the ridge and birdsmouth cuts.
⚠️ Measure on a level line - a tape on the slope reads too shallow.
How to use this roof pitch calculator
- Measure the vertical rise over a level run (12 in is the roofing standard) and enter both.
- Enter the roof run - the horizontal distance from the wall to the ridge - in feet for the rafter length.
- Read the pitch, angle, slope factor and rafter length, and check the material-suitability status.
How roof pitch, angle and rafter length are calculated
Roof pitch is written as rise-in-12 - the vertical rise for every 12 inches of horizontal run - so a roof that rises 6 inches over 12 is a 6:12 pitch. The angle is arctan(rise ÷ run). The slope factor is the length of the sloping roof per unit of horizontal run, √(rise² + run²) ÷ run, and it doubles as the roof-area multiplier: multiply the flat plan area by it to get the actual roof surface. Rafter length is the slope factor times the roof run. The percent grade, rise ÷ run × 100, is the same slope expressed the way ground and drainage are measured. The suitability check uses IRC R905 shingle slopes - 2:12 minimum with a double underlayment, 4:12 for a standard install.
Frequently asked questions
- What is my roof pitch?
- Roof pitch is the rise in inches for every 12 inches of horizontal run. Measure how far the roof rises over a level 12-inch run: a 6-inch rise is a 6:12 pitch, which is about 26.6°. Enter your rise and run above to read the pitch, angle and grade exactly.
- What is the minimum roof pitch for shingles?
- Asphalt shingles need a minimum slope of 2:12, and only with a double layer of underlayment; at 4:12 and above a standard single-underlayment install is allowed. Below 2:12 you should use a low-slope membrane - EPDM, TPO or built-up roofing - instead of shingles. The calculator flags which band your pitch falls into.
- How do I calculate rafter length from pitch?
- Multiply the horizontal roof run by the slope factor √(rise² + run²) ÷ run. For a 6:12 pitch the factor is about 1.118, so a 12-foot run needs a rafter of about 13.4 feet - before adding for the overhang and the ridge and birdsmouth cuts. Enter your run above and the calculator does it for you.
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