Roof Pitch Calculator
Calculate roof pitch ratio, angle in degrees, and rafter length from rise and run measurements.
Roof pitch comes up any time you're calculating materials for a roofing project or building a shed. The pitch multiplier is the number I use most, multiply your floor plan area by the multiplier to get the actual roof surface area, which is always larger than the footprint.
Roof pitch is expressed as rise-over-run in a 12-inch run. A 6:12 pitch rises 6 inches for every 12 inches of horizontal run. The angle is about 26.6 degrees.
How roof pitch is expressed
US roof pitch uses a ratio of rise to run, where run is always 12 inches. A 6:12 pitch rises 6 inches for every 12 horizontal inches. Common pitches range from 3:12 (low slope) to 12:12 (45-degree angle). Pitches below 2:12 require special roofing materials as they don't shed water well. Pitches above 9:12 are steeper and require different installation techniques.
Rafter length calculation
Rafter length is calculated using the Pythagorean theorem. For a 12-foot run with 6:12 pitch, the rise is 6 feet (6 inches per foot × 12 feet run). Rafter length = √(run² + rise²) = √(144 + 36) = √180 ≈ 13.4 feet. Add the rafter tail (overhang) length to this for the total cut rafter length. This calculator gives the structural rafter length from ridge to wall plate.
The roof area multiplier
The multiplier converts floor plan area to actual roof surface area. A 6:12 pitch has a multiplier of about 1.118, a 1,000 sq ft house footprint has roughly 1,118 sq ft of roof surface. Use this when calculating shingles, underlayment, or roofing material quantities. The steeper the pitch, the larger the multiplier.
Common pitch applications
2:12 to 3:12, very low slope, requires rolled roofing or membrane. Common on additions and sun rooms. 4:12 to 6:12, standard residential pitch. Works with all common shingle types. 7:12 to 9:12, steep pitch. More dramatic appearance, better water shedding, more complex installation. 10:12 to 12:12, very steep. Sometimes requires safety equipment for installation.
Frequently asked questions
How do I measure an existing roof pitch?
Use a level and a tape measure. Hold the level horizontally against the roof surface. At the 12-inch mark on the level, measure vertically down to the roof surface. That vertical measurement (in inches) is the rise, giving you the X in X:12 pitch. A pitch gauge tool (available at hardware stores) makes this easier.
Does pitch affect shingle quantity?
Yes, the roof area multiplier accounts for this. A steeper pitch means more actual roof surface per square foot of footprint, requiring more shingles. Always use actual roof surface area (footprint × multiplier) when ordering materials, never the floor plan area alone.
What pitch do I need for proper drainage?
The general minimum for asphalt shingles is 2:12 (with special low-slope installation techniques) or 4:12 for standard installation. Metal roofing can work as low as 1:12 with proper installation. Flat roofs (technically 1/4:12) require membrane roofing systems designed for near-horizontal drainage.