About this tool
Roof Pitch Calculator is a free, in-browser tool that converts between every way a roof slope is described: the rise-over-run ratio, the builder's x-in-12 pitch, the angle in degrees, the slope (multiplier) factor, and the percent grade. Pick what you know from the selector — rise and run, an angle, a grade, or a pitch — and the tool fills in the rest instantly. Nothing leaves your browser.
The slope ratio is rise ÷ run. From it, the pitch is 12 × ratio expressed as x:12, the angle is arctan(ratio) converted from radians to degrees, the slope factor is √(1 + ratio²) — the number you multiply the flat footprint by to get true roof area or rafter length — and the grade is ratio × 100 percent. When you enter an actual rise and run, it also reports the rafter length √(rise² + run²) for that run.
Use it to read a blueprint's 6:12 callout as an angle, to translate a surveyor's percent grade into a pitch for ordering shingles, or to check that a proposed slope drains and matches your material's minimum. It complements a roofing material calculator, which handles coverage and waste, by nailing down the geometry first.
Frequently asked questions
What does a pitch like 6:12 mean?
It means the roof rises 6 units for every 12 units of horizontal run. The first number is the rise per 12 of run, so 6:12 is a moderate slope and 12:12 is a 45° roof. The tool shows this alongside the equivalent angle, grade and slope factor.
How is the angle worked out from rise and run?
The angle is the arctangent of rise ÷ run, converted from radians to degrees (multiply by 180/π). For a 6:12 pitch that is arctan(0.5) ≈ 26.57°. You can also enter the angle directly and read off the matching pitch and grade.
What is the slope factor for?
It is √(1 + (rise/run)²) — the multiplier that turns a roof's flat footprint area into its true sloped area, and a horizontal run into rafter length. A 6:12 roof has a slope factor of about 1.118, so its surface is roughly 12% larger than the plan area beneath it.
Percent grade versus pitch — how do they relate?
Grade is simply the rise/run ratio written as a percent, so a 6:12 pitch is a 50% grade. Roads and drainage plans use grade, while roofers use x:12 pitch; this tool converts freely between them, all computed locally in your browser.
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