Use this free roof pitch calculator to find pitch angle, rafter length, vertical rise, and total roof area — instantly.
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Note: "Vertical Rise" is measured from the top of the wall plate to the ridge board — it is not the total building height above grade, which would also require wall height (not collected by this tool).
Choose your input method from the three tabs: enter your building span and pitch (most common), your rise and run, or your roof angle in degrees. Add the building length and eave overhang, then click Calculate. The tool instantly returns pitch ratio, rafter length, vertical rise, roof angle, and total roof area.
Getting the roof pitch right isn't just a geometry exercise — it has real consequences for cost, materials, and code compliance. A 6/12 pitch means for every 12 inches of horizontal run, the roof rises 6 inches vertically, creating a 26.6° angle. That's considered a standard residential pitch across most of North America.
Consider a 24-foot-wide, 40-foot-long house with a 6/12 pitch. Each common rafter measures about 13.4 feet before any overhang. Add a 12-inch eave overhang and the rafter grows to roughly 14 feet 6 inches (14.53 ft) — multiply that across 80+ rafters on a typical house and material costs add up fast if you miscalculate even a few inches.
Pitch also drives water runoff, snow load capacity, attic livability, and which roofing material is even allowed. Low-slope roofs under 3/12 typically require different underlayment and materials than standard-slope roofs, while anything above 9/12 usually calls for a safety harness during installation. Knowing your exact numbers before ordering lumber or shingles can save hundreds of dollars and prevent surprises on the job site.
The math behind roof pitch is straightforward trigonometry. Here are the core formulas:
The rafter factor is simply the Pythagorean theorem applied per foot of run: a triangle with a 12-inch base and a "pitch"-inch height has a hypotenuse of √(12²+pitch²) inches, which becomes the sloped length of rafter needed for every foot of horizontal run.
Let's calculate a real roof: a house with a 24-foot span, 6/12 pitch, 40-foot building length, and a 12-inch eave overhang — entered on the Span & Pitch tab above.
Plug those same numbers into the calculator above and you'll see an identical result — pitch 6.00/12, angle 26.57°, and a rafter length (with overhang) of 14 ft 6.4 in.
Most residential homes use pitches between 4/12 and 9/12 (18.4° to 36.9°). A 6/12 pitch is extremely common — it sheds water well, is safe to work on with care, and offers usable attic space. Pitches below 3/12 are considered low-slope and above 12/12 are considered steep-slope, each requiring different roofing materials and installation methods.
In common usage they're interchangeable, but technically "pitch" is the ratio of rise to total span, while "slope" is rise over run expressed as x/12. The roofing industry almost universally uses the x/12 slope format and calls it pitch — so 6/12 means 6 inches of rise per 12 inches of run. This calculator uses that industry-standard convention.
Use a level and a tape measure. Hold a 12-inch level horizontally against the roof surface, then at the 12-inch mark measure straight down to the roof — that vertical measurement is your pitch. For example, 6 inches down means a 6/12 pitch. You can also use a digital angle finder and convert with pitch = 12 × tan(angle).
This tool calculates common rafter lengths for standard gable roofs. Hip roofs need separate hip and valley rafter calculations because those rafters run diagonally in plan as well as in slope. As a rough guide, the hip-to-common rafter ratio is √2 only on a flat roof; on a 6/12 pitch it's closer to 1.34 and it keeps shrinking as pitch increases, so use a dedicated hip rafter calculator for precise cuts.
Most roofers consider anything above 6/12 to require extra caution, and above 9/12 usually requires fall-protection equipment like a harness and roof anchors. Steeper pitches also need slip-resistant footwear and roof jacks or toe boards for safe footing during installation or repair.
Overhang adds proportionally to rafter length based on the rafter factor, not a flat amount. For a 6/12 pitch, a 12-inch (1 ft) overhang adds about 1.12 ft of rafter length; for a steeper 12/12 pitch, that same 1 ft of overhang adds about 1.41 ft, since the steeper slope stretches every foot of horizontal run further.
Calculator by Toolwick