Roof Pitch Calculator

Roof pitch is the ratio of vertical rise to horizontal run, expressed as rise-over-12. To calculate: measure how many inches the roof rises for every 12 inches of horizontal distance. A roof that rises 6 inches for every 12 inches of run has a 6/12 pitch. For an exact calculation, divide the total rise by the total run, then multiply by 12 to express it in the standard format. The calculator above accepts rise and run directly, or you can enter the angle in degrees and get the pitch ratio back.

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 a complete list of common pitches and their degree equivalents.

Place a 12-inch 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 is your rise. A 6-inch 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 using a level and tape measure if the rafters aren't accessible.

Multiply the building footprint (length × width) by the pitch multiplier shown above. The pitch multiplier accounts for the extra surface area added by the slope — a 6/12 pitch produces a multiplier of 1.118, so a 1,200 sq ft footprint becomes 1,341 sq ft of roof surface. For a complete roof area calculation including waste factor, use our Roof Square Footage Calculator (linked in the related calculators section).

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 for installation. Steeper pitches (8/12 to 12/12) are common in regions with heavy snow because they shed snow load more effectively. Lower pitches (3/12 to 4/12) are common in modern architecture, ranch-style homes, and additions.

The roofing industry classifies pitch into three categories. Low-slope: 0/12 to 3/12 (0° to 14°) — typically requires special membrane roofing rather than shingles. Conventional/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°+) — requires safety equipment for installation and may need specialty materials. The 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, which is the same as the pitch multiplier. For a 6/12 pitch, that's 13.4 inches of rafter per 12 inches of horizontal run. For a 20-foot horizontal run, you'd need 20 × 13.4/12 = 22.4 feet of rafter (before adding overhangs and seat cuts). The calculator above shows rafter length per foot in the results — multiply by your run to get 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 input quality. Measure rise and run precisely (level + tape measure works), confirm with multiple measurements if possible, and account for any non-uniform sections of the roof separately. For complex roofs with multiple pitches, calculate each plane separately and combine.

Pitch ratios, angles, and percentages are unit-neutral — they work the same whether you measure in inches or centimeters. For rise and run inputs, use any unit you prefer (the calculator computes the ratio, not absolute lengths). The rafter-per-foot output shows centimeters in parentheses for metric reference.