ROOF PITCH
About this calculator
This roof pitch calculator converts a measured rise and run into the four ways pitch is expressed: the trade format (X in 12), the angle in degrees, the slope as a percentage, and the slope multiplier used to convert plan-view square footage into actual roof surface area for shingle ordering. Standard residential pitches run from 4/12 (low) through 12/12 (steep). Anything under 2/12 is considered low slope and needs a membrane roof, not asphalt shingles.
How to use this calculator
Measure the rise (vertical distance) at exactly 12 inches of run (horizontal distance) for the trade-format X/12 pitch. Use a level: hold it horizontal against the rafter or roof surface, mark 12 inches from the level's tip, then measure the vertical gap at that mark.
If you measured rise over a different run distance (say 24 inches or 36 inches), enter both — the calculator normalizes the result to the 12-inch convention. The result includes the pitch in /12 format, the angle in degrees, slope as a percentage, and the slope multiplier used to convert plan-view square footage to actual roof surface area for shingle ordering.
Worked example
A roof rises 6 inches over 12 inches of horizontal run:
Pitch = (6 × 12) ÷ 12 = 6/12. Angle = arctan(6/12) = 26.57°. Slope % = 6 ÷ 12 × 100 = 50%. Slope multiplier = √(36 + 144) ÷ 12 = 1.118.
For a roof with a 1,200 ft² footprint (top-down view) and 6/12 pitch: actual roof surface = 1,200 × 1.118 = 1,341 ft². Shingle ordering uses the surface area, not the footprint.
A 4/12 roof: angle 18.4°, slope 33%, multiplier 1.054. A 9/12 roof: angle 36.9°, slope 75%, multiplier 1.25. A 12/12 roof: angle 45°, slope 100%, multiplier 1.414.
If you measured 12 inches of rise over 36 inches of run (took a wider span for accuracy): pitch = (12 × 12) ÷ 36 = 4/12. Same roof, easier measurement.
Common mistakes & waste factors
Mixing up rise and run. Rise is vertical; run is horizontal. A 6/12 pitch rises 6 inches vertically for every 12 inches you move horizontally — not the other way around.
Measuring along the slope instead of horizontally. The 12-inch run is the horizontal distance, not the slope distance. Measuring along the slope gives a steeper-looking pitch than reality.
Using degrees and pitch interchangeably. A 6/12 pitch is 26.57°, not 6°. The trade convention is /12; degrees are for engineering and architecture work.
Forgetting that low-slope roofs need different roofing. Below 2/12 pitch, asphalt shingles aren't allowed by code — water can back up under the tabs in driving rain. Use modified bitumen, EPDM, or TPO membrane instead.
Rules of thumb
Pitch in /12 = (rise × 12) ÷ run.
Common pitches: 4/12 = 18.4° · 6/12 = 26.6° · 9/12 = 36.9° · 12/12 = 45°.
Slope multipliers: 4/12 = 1.054 · 6/12 = 1.118 · 8/12 = 1.202 · 12/12 = 1.414.
Below 2/12 = low slope, requires membrane roofing.
Above 4/12 = standard residential, asphalt shingles and standard install methods work.
Above 9/12 = steep, requires toe-boards or scaffolding for safe install.
Common questions
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