Calculate pitch angle, rafter length, ridge height, and total roof area — instantly.
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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, ridge height, 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 in most of North America.
Consider a 24-foot wide, 40-foot long house with a 6/12 pitch: each rafter is about 13.4 feet long before overhang. With a 12-inch overhang, you're looking at roughly 14.4 feet of actual rafter stock per side — multiply that across 80+ rafters and material costs add up fast if you miscalculate.
Pitch also drives water runoff, snow load capacity, and attic livability. Pitches below 3/12 often require special low-slope roofing materials. Pitches above 9/12 require safety harnesses for installation. Knowing your exact numbers before buying materials can save hundreds of dollars and prevent dangerous surprises on the job site.
The math behind roof pitch is straightforward trigonometry. Here are the core formulas:
The rafter factor accounts for the slope: actual rafter length per 12" run = √(12² + pitch²) / 12. This factor is then multiplied by the run (including overhang in feet) to get total rafter length.
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 walk 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 the total span (rise/span), while "slope" is rise over run (rise/run × 12). The roofing industry almost universally uses slope in the "x/12" format and calls it pitch — so 6/12 means 6 inches of rise per 12 inches of run. This calculator uses the industry-standard convention.
Use a level and a tape measure. Hold a 12-inch level horizontally against the roof surface. At the 12-inch mark, measure straight down to the roof. That vertical measurement is your pitch. For example, if you measure 6 inches down, your roof is a 6/12 pitch. You can also use a digital angle finder or smartphone level app directly on the roof surface and convert the angle: pitch = 12 × tan(angle).
This tool calculates common rafter lengths for standard gable roofs. Hip roofs have additional hip and valley rafters that require separate calculations. The roof area result does account for the sloped surface area correctly for any rectangular footprint, but you'll need a hip rafter calculator to determine hip rafter lengths specifically. For a quick estimate, hip rafter length ≈ common rafter length × √2 (for a 45° hip angle).