Roof Pitch Calculator

Measure two things and you're done. Rise is how far the roof climbs vertically; run is the horizontal distance that rise is spread over. Enter both in the same unit — inches work best — and the calculator returns your roof pitch as a rise-over-run ratio (the classic x:12 format), the roof angle in degrees, the slope as a grade percentage, and the rafter length that spans that triangle. The most reliable field method is a 12-inch level: hold it dead level against the roof, mark 12 inches out, then measure straight down from that mark to the roof surface — that vertical drop is your rise over a 12-inch run. If you already know the total rise and total run of the whole roof, enter those instead; the ratio comes out identical.

Roof pitch describes how steep a roof is, written as rise over run. In the United States the run is fixed at 12, so a '6/12 pitch' means the roof rises 6 inches for every 12 inches it travels horizontally. That is the same slope whether you measure a 6-foot rise over a 12-foot run or 6 inches over 12 inches — pitch is a ratio, so the units cancel out. People often ask *what angle is a 6/12 roof pitch* or how to convert *roof pitch to degrees*; the answer is simple trigonometry — the angle is the arctangent of rise divided by run. A 6/12 roof works out to about 26.6°, a 4/12 to 18.4°, and a 12/12 (rise equals run) to a full 45°. Grade percentage says the same thing another way — rise divided by run times 100 — so 6/12 is a 50% grade. Roofers, inspectors, and shingle makers all lean on these numbers: most asphalt shingles need at least a 4/12 pitch, and anything below 2/12 is treated as a low-slope roof that needs different materials.

Pitch (x:12) = (rise / run) x 12
Angle = arctan(rise / run)
Grade % = (rise / run) x 100
Rafter length = sqrt(rise^2 + run^2)

Rafter length comes straight from the Pythagorean theorem: the rise and run are the two legs of a right triangle and the rafter is the hypotenuse. This is exactly the math a rafter length calculator uses — if you know rise and run, you know the sloped length you need to cut.

Example 1 — a standard house. A roof rises 6 inches over a 12-inch run. Pitch is 6/12, the angle is 26.6°, the grade is 50%, and each foot of run needs about 13.4 inches of rafter. Example 2 — a low-slope porch. Rise 3 over run 12 gives a 3/12 pitch, 14.0°, a 25% grade — below the 4/12 shingle minimum, so you'd pick a rolled or metal roof. Example 3 — measuring the whole roof. If the ridge sits 8 feet above the top plate and the run from wall to ridge is 16 feet, enter rise 8 and run 16: that is a 6/12 pitch again (8 / 16 x 12 = 6).

- Use the same unit for rise and run. Mixing inches and feet is the number-one error; the ratio only works when both legs share a unit.
- Run is always horizontal, never along the slope. Measuring up the roof surface gives you the rafter length, which overstates the pitch.
- A 12-inch level is the fast field method: level it, mark 12 inches, measure the drop — that drop is your rise on a 12 run, giving pitch directly.
- For a whole-roof measurement, run is half the span (wall to ridge), not the full building width.
- Steeper isn't always better — pitches above 9/12 usually need roof jacks or scaffolding and cost more to install.