Roof Pitch Calculator
Convert roof rise and run into pitch (X:12), angle in degrees, slope percent, and rafter length. Free roof pitch calculator for builders and DIYers.
How to use this roof pitch calculator
Roof pitch is the steepness of a roof, almost always spoken as “X-in-12”: inches of rise for every 12 inches of horizontal run. Building codes, shingle warranties, and snow-load tables all key off that number. Low slopes (under 4:12) often need special membranes; conventional asphalt shingles are happiest from about 4:12 to 9:12.
Enter rise and run from a level and tape, or from plans. Add span and overhang to estimate common-rafter length with the pitch factor (the hypotenuse of a 12-in run triangle). This does not replace a licensed rafter book for birdsmouths, ridge thickness, or hip/valley lengths, but it is the right first number for material runs and ladder setup.
Formula
Worked example
A 6-in rise over a 12-in run is a 6:12 pitch, 26.57°, and 50% slope. On a 24 ft span with 12 in overhang, each common rafter is about 13.42 ft.
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Frequently asked questions
How do I calculate roof pitch?
Divide rise by run. Multiply that ratio by 12 to write pitch as X:12. A 6 inch rise on a 12 inch run is 6:12.
What is a 6/12 pitch in degrees?
arctan(6/12) = arctan(0.5) ≈ 26.57°.
How do I find rafter length from pitch?
Multiply the run of the rafter (half-span plus overhang) by √(1 + (rise/run)²). This calculator does that when you enter span.
Is 4/12 a low slope roof?
4:12 is the usual cutoff between low-slope and conventional steep-slope shingle roofs. Below 3:12 is typically membrane territory.
Pitch vs slope vs angle?
Pitch is X:12, slope is a percent, and angle is degrees. They all describe the same triangle.
What pitch is best for shingles?
Conventional asphalt shingles are happiest from about 4:12 to 9:12. Lower slopes often need special membranes.
Results are estimates for informational purposes and are not financial, legal, or professional advice. Last reviewed 2026-08-20.