Roof Pitch Calculator
Convert roof rise and run into a pitch ratio, the slope angle in degrees and the area multiplier.
How it works
pitch = rise ÷ run × 12 · angle = arctan(rise ÷ run) · area multiplier = √((rise ÷ run)² + 1)
Roof pitch is stated as rise over a standard 12-inch run, so a 6-inch rise per foot is 6/12. The slope angle is the arctangent of rise over run. The area multiplier — the diagonal length per unit of run — is what converts a flat footprint into true sloped area for ordering shingles and underlayment.
Worked example
A 6 in rise over a 12 in run is a 6/12 pitch: arctan(6 ÷ 12) = 26.6°, and the multiplier is √(0.5² + 1) = 1.118. A 40 × 30 ft roof at this pitch covers 1,200 × 1.118 ≈ 1,342 sq ft.
Frequently asked questions
What counts as a walkable roof?
Up to about 6/12 (roughly 27°) most people can walk with care in good shoes. Past 8/12 it gets genuinely dangerous and calls for roof jacks, harnesses or a pro. Never judge safety by feel alone on a wet or mossy roof.
What pitch do I need to shed water?
Asphalt shingles want at least 2/12, and below 4/12 they need a doubled underlayment or a sealed membrane. Truly flat roofs use rolled or membrane systems, not shingles.
Why does pitch change how much material I buy?
A steeper roof has more surface over the same footprint. The area multiplier captures that — a 12/12 roof needs 41% more shingles than a flat one covering the same ground.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.