How to use the roof pitch calculator
- Hold a level horizontally against the roof or a rafter and mark 12 in.
- Measure straight up (or down) from that mark to the roof surface — that is the rise.
- Enter rise and run, or switch to angle if you used a digital gauge.
- Optionally enter the horizontal run (half the span) to get a rafter length.
Formula
Pitch = rise ÷ run × 12. Angle = arctan(rise ÷ run). Percent slope = rise ÷ run × 100. Pitch multiplier = √(1 + (pitch ÷ 12)²) — multiply a plan-view area by it to get the sloped roof area.
Worked example
A rise of 6 in over 12 in is a 6/12 pitch, an angle of 26.57°, a 50% slope and a multiplier of 1.118. A 1,200 sq ft footprint therefore has about 1,342 sq ft of roof surface.
Tips for using the roof pitch calculator
- Measure from inside the attic when possible; it is safer than standing on the roof.
- Use the pitch multiplier to turn footprint area into true roof area for ordering materials.
- Check minimum pitch for your roofing: asphalt shingles need at least 2/12 with extra underlayment.
Roof Pitch Calculator FAQs
What is a 4/12 roof pitch in degrees?
4/12 is 18.43°, a 33.3% slope, with a multiplier of 1.054.
What is the minimum pitch for asphalt shingles?
The International Residential Code (R905.2.2) allows asphalt shingles on slopes of 2/12 or more, with double underlayment required between 2/12 and 4/12. Manufacturers' instructions and your local code govern.
How do I measure pitch without going on the roof?
Measure a rafter tail or gable rake from a ladder, or from inside the attic against a rafter. Avoid climbing a roof without fall protection.
What pitch is a "steep" roof?
Roofers commonly treat slopes over about 8/12–9/12 as steep because they need roof jacks or harnesses, which raises labor cost.
