Roof Pitch Calculator

Roof pitch describes how steep a roof is, written as a ratio of rise to run— the vertical rise for every 12 inches of horizontal run. A "6:12" roof rises 6 inches for every 12 inches it runs horizontally, which is why pitch is always expressed relative to a run of 12 in US construction.

The easiest way to measure an existing roof's pitch is from inside the attic. Hold a torpedo level flat against the underside of a rafter so it reads perfectly level, then measure exactly 12 inches out along the level. At that 12-inch mark, measure straight down from the level to the top of the rafter — that distance, in inches, is your rise. Run the numbers through the calculator above to get the pitch, angle, and slope in one step.

Once you know the rise and run, the roof's angle from horizontal follows directly from basic trigonometry:

Angle = arctan(Rise ÷ Run) × (180 ÷ π)

Worked example: a 6/12 roof has a rise of 6 inches over a run of 12 inches. arctan(6 ÷ 12) = arctan(0.5) = 0.4636 radians, and 0.4636 × (180 ÷ π) ≈ 26.57°. The same formula also gives you slope as a percentage — rise ÷ run × 100 — so a 6/12 pitch is a 50% slope.

A sloped rafter is always longer than the flat horizontal distance it spans, because it's the hypotenuse of a right triangle formed by the rise and the run. The Pythagorean theorem gives the exact length:

Rafter length = √(Rise² + Run²)

Worked example: for a 6/12 pitch, every 12 inches of run needs √(6² + 12²) = √180 ≈ 13.42 inches of rafter — a multiplier of about 1.118 per foot of run. So if the horizontal distance from the wall plate to the ridge is 15 ft, the rafter length is roughly 15 × 1.118 ≈ 16.77 ft, or about 16 ft 9 in, before adding any overhang past the wall.

As general guidance rather than a building code, roofers typically group pitches into a few bands:

Low-slope roofs (under 3/12)shed water slowly and usually need a membrane roofing system or a manufacturer's low-slope shingle application, since standard asphalt shingles aren't rated for pitches that shallow — always check the shingle manufacturer's installation guidance before building this low.

Walkable pitches (roughly 4/12–6/12) are the most common on US homes. They shed water and snow well while still being safe enough to walk for most roofers, and they work with standard shingles, tile, and metal roofing without extra steps.

Steep pitches (8/12 and above) shed water and snow very effectively and are often chosen for their appearance, but they generally require fall protection to work on safely and can add material and labor cost due to the extra rafter length and steeper working conditions.