Calculator What does the roof pitch calculator calculate?
Divide the vertical rise by the horizontal run, then express the result as inches of rise per 12 inches of run. For example, a 6-inch rise over 12 inches of horizontal run is a 6:12 pitch. This calculator also converts the same slope to percent and degrees, and uses the rise and run to estimate the diagonal rafter length.
Enter rise and run in the same unit because the roof pitch calculator treats their ratio as unitless. Rise is the vertical change and run is the horizontal projection for the same roof segment. Measure from consistent endpoints rather than combining a total rise with a partial run. The diagonal output represents the straight hypotenuse between those endpoints; it is not automatically a purchasable or cut rafter length.
How does the roof pitch calculator formula work?
Slope ratio is rise ÷ run. Pitch per twelve is rise ÷ run × 12. Slope percent is rise ÷ run × 100, and roof angle is arctan(rise ÷ run). For a straight rafter without allowances, use rafter length = √(rise² + run²). Use horizontal run, not the sloped roof surface, in these formulas.
All four outputs come from the same ratio or right triangle. The roof pitch calculator normalizes that ratio to a 12-unit run for pitch notation, to 100 for percentage, and through inverse tangent for degrees. The Pythagorean step uses the original rise and run, so its diagonal retains their unit. As a check, multiplying the pitch number by run/12 should recover rise, apart from rounding.
Roof pitch calculator worked example
A roof rises 4 feet over a horizontal run of 12 feet. The slope ratio is 4 ÷ 12 = 0.3333, which converts to 4:12 and 33.3%. The angle is about 18.4 degrees. The diagonal distance is √(4² + 12²), or about 12.65 feet before any ridge, overhang, birdsmouth, or connection allowances.
The example also shows why units must match. Entering 4 inches of rise and 12 feet of run without conversion would describe a much flatter slope than 4:12. In the roof pitch calculator, converting both measurements to feet gives 0.333/12, while converting both to inches gives 4/144; those ratios match. Doubling both rise and run leaves pitch, percentage, and angle unchanged but doubles the straight diagonal length.
What are the limits of the roof pitch calculator?
Roof geometry is not a structural design or code-compliance determination. Actual framing needs allowances for overhangs, ridge details, bearing cuts, member size, loads, wind, snow, drainage, roofing material, flashing, and local requirements. Measure an existing roof safely and do not rely on this page for cut lists or permitting. Consult qualified building and design professionals for a specific project.
The roof pitch calculator models one straight right-triangle section and does not represent hips, valleys, unequal slopes, curved roofs, or changes in pitch. It has no fields for eaves, ridge thickness, cuts, or member bearing. Do not add those items as arbitrary percentages to the diagonal; use the approved framing geometry and details. Remote or ground-based measurements can also contain error, so consequential work requires field verification by a qualified person using safe access methods.
How does the roof pitch calculator compare with related calculators?
Pitch convention expresses rise per 12 units of run, while slope percentage expresses rise per 100 units of run. Both describe the same geometry. Plan area differs from actual roof surface area on a slope; the latter is larger. Use the square foot calculator for plan area and apply verified roof measurements when ordering roofing materials.
Use the roof pitch calculator when the known measurements are vertical rise and horizontal run. Use a Pythagorean calculation when only a missing right-triangle side is needed, and a surface-area method when material coverage is the goal. Converting pitch to degrees does not add information; it presents the same slope in another convention.
Roof pitch calculator questions
Is 6:12 the same as 6 degrees?
No. A 6:12 pitch means six units of rise for 12 horizontal units and corresponds to roughly 26.6 degrees.
The roof pitch calculator displays both values so the ratio notation and angular notation can be checked without treating their numbers as interchangeable.
Can I measure run along the roof surface?
No. Run is the horizontal distance. Measuring along the roof surface gives the rafter length and will produce the wrong pitch.