Roof Pitch, Slope & Gradient Calculator

Three interchangeable ways: rise-in-12 — the US standard — written x:12 (a 4:12 pitch means 4 inches of rise for every 12 inches of run), degrees (the angle from horizontal), and percent (rise ÷ run × 100). A 4:12 pitch is about 18.4°, or a 33% slope; a 12:12 pitch is 45°. The calculator converts freely between all three.

Rafter = horizontal half-span ÷ cos(pitch angle). A building 24 ft wide (12 ft from wall plate to ridge, horizontally) at a 4:12 pitch (18.4°) needs 12 ÷ cos 18.4° ≈ 12.6 ft (3.85 m) rafters, and the ridge rises 12 × tan 18.4° ≈ 4 ft (1.22 m) above the plates. Add the eaves overhang and birdsmouth allowance on top.

Hold a level horizontally against the rafter or roof surface, measure 12 in along the level, then measure vertically down to the surface — that vertical figure in inches is the rise, giving the pitch directly as rise:12. A 4 in drop over 12 in is a 4:12 pitch (18.4°). Enter that ratio and the calculator reads off the angle and percent.

It depends on the covering: standing-seam and profiled metal panels commonly go down to about 1:12–2:12 (5–10°) depending on the profile and seams, asphalt shingles generally want a 2:12 minimum (with special underlayment below 4:12), and concrete tiles typically need 4:12–6:12 (17–26°). Below the covering’s minimum, wind-driven rain gets under the laps. Always use the manufacturer’s stated minimum for the exact product, not a generic figure.

Yes — rafters lengthen, the covering area grows (area = plan area ÷ cos pitch), and scaffolding and labor rise with the pitch. A 12:12 roof (45°) has over 40% more surface than a flat one covering the same plan. Against that, steep roofs shed water and dirt better, resist leaks and can add attic space — the sweet spot is usually the covering’s comfortable range.