Roof Pitch Calculator
Pitch, angle, rafter length and the area multiplier.
How it works
Roof pitch is conventionally written as rise over a run of 12 — a "6 in 12" roof climbs 6 units for every 12 horizontal. The angle is arctan(rise/run), and the rafter is the hypotenuse.
The area multiplier is the number that matters when ordering materials. Roof area is the plan footprint multiplied by √(1 + (rise/run)²), so a 6:12 roof has 11.8% more surface than the ground it covers. Ordering shingles from the footprint is how people end up one bundle short.
Practical thresholds: below 2:12 most shingles are not rated and you need a membrane; above 6:12 you are working on roof jacks rather than standing on it.