Roof Pitch & Slope Calculator

Roof pitch is the ratio of vertical rise to horizontal run, expressed as inches of rise per 12 inches of run. A 6/12 pitch rises 6 inches (15 cm) for every 12 inches (30 cm) of horizontal distance. To calculate it, measure the rise from a level point to the underside of the roof at 12 inches horizontally, or use a digital level with a pitch reading. The calculator above converts between rise/run ratios, degrees, and percentage slope. Enter any one and read the others.

Take the arctangent of rise divided by run. For a 6/12 pitch: arctan(6/12) = arctan(0.5) = 26.6°. Common conversions: 4/12 = 18.4°, 6/12 = 26.6°, 8/12 = 33.7°, 12/12 = 45° (because rise equals run), 24/12 = 63.4°. Use the reference chart on this page for the full list of common pitches and their degree equivalents.

Place a 12-inch (30 cm) level horizontally against a roof rafter, with one end touching. From the other end, measure straight down to the rafter. That vertical distance in inches (or cm) is your rise. A 6-inch (15 cm) drop means 6/12 pitch. The safest way is to measure from inside the attic against an exposed rafter rather than working on the roof itself. You can also measure from the gable end of the house with a level and tape if the rafters aren't accessible.

Multiply the building footprint (length × width) by the pitch multiplier shown above. The multiplier captures the extra surface area added by the slope. A 6/12 pitch (26.6°) gives a multiplier of 1.118, so a 1,200 sq ft (111.5 m²) footprint becomes 1,341 sq ft (124.6 m²) of roof surface. For a full roof area calculation that includes a waste factor, use the Roof Square Footage Calculator (linked in related calculators).

Most residential roofs in the US fall between 4/12 and 9/12 (18° to 37°). The single most common pitch is 6/12 (26.6°), which balances aesthetics, weather performance, and walkability during installation. Steeper pitches (8/12 to 12/12) are common in heavy-snow regions because they shed snow load better. Lower pitches (3/12 to 4/12) show up in modern architecture, ranch-style homes, and additions.

The roofing industry splits pitch into three categories. Low-slope: 0/12 to 3/12 (0° to 14°), which usually needs membrane roofing rather than shingles. Conventional or medium slope: 4/12 to 9/12 (18° to 37°), the standard residential range that uses asphalt shingles. Steep slope: 9/12 and above (37°+), which needs safety equipment for installation and may need specialty materials. Minimum pitch for asphalt shingles is generally 2/12, though 3/12 is the safer minimum.

Rafter length per foot of run = √(rise² + 12²) ÷ 12, the same as the pitch multiplier. For a 6/12 pitch (26.6°), that's 13.4 inches (34 cm) of rafter per 12 inches (30 cm) of horizontal run, or about 1.12 m of rafter per meter of run. For a 20-foot (6 m) horizontal run, you'd need 20 × 13.4/12 = 22.4 feet (6.7 m) of rafter (before overhangs and seat cuts). The calculator above shows rafter per foot (imperial) or per meter (metric). Multiply by your run for total rafter length.

The math is exact for the geometry it models. Pitch ratio, angle, percentage, and rafter length per unit run all derive from basic trigonometry. Real-world accuracy depends on the inputs. Measure rise and run precisely (a level and tape measure works), confirm with multiple measurements if possible, and account for non-uniform sections separately. For complex roofs with multiple pitches, calculate each plane separately and combine.

Yes. Pick Metric in the unit selector at the top and the rise/run inputs switch to centimeters, with the featured output row swapping from X/12 (imperial) to degrees (metric). All four input modes work in both systems. Rise/run units differ by system, while ratio, angle, and percent are unitless or universal. The rafter output reads as in/ft in imperial and cm/m in metric. Your unit choice sticks across pages and tabs via localStorage.

In casual conversation and most modern usage, yes. Roofers, builders, and homeowners use the terms interchangeably for how steep a roof is. Technically, slope is the rise per 12 inches of run (the 'x/12' notation like 6/12), while architects historically defined pitch as rise over the entire building span (1/6, 1/4, etc.). Most online searches for 'roof slope calculator' want exactly the same thing as 'roof pitch calculator': the rise/run ratio, angle in degrees, and percentage. This calculator handles all those formats.