Roof pitch calculator: pitch, angle, and actual roof area

How do you convert roof pitch to degrees?

The angle is the arctangent of rise divided by run. For a 6:12 roof that is arctan(6/12), which is 26.57 degrees. Percentage slope is simply rise divided by run multiplied by 100, so 6:12 is a 50% slope.

Pitch	Degrees	Percentage	Area multiplier
3:12	14.04°	25%	1.031
4:12	18.43°	33%	1.054
6:12	26.57°	50%	1.118
8:12	33.69°	67%	1.202
10:12	39.81°	83%	1.302
12:12	45.00°	100%	1.414

Why is the roof bigger than the floor beneath it?

Because a slope is longer than the horizontal distance it covers. The ratio between them is the pitch multiplier, and it is exactly √(1 + (rise/run)²) by the Pythagorean theorem.

At 6:12 the multiplier is 1.118, so the roof surface is about 12% larger than the footprint. At 12:12 it is 1.414, or 41% larger. Ordering shingles against the floor area rather than the sloped area is one of the most common ways to end up short, and on a steep roof it is short by a lot.

What is the minimum pitch for each roofing material?

This is a genuine constraint rather than a preference. Below the minimum, water moves too slowly and can drive back under the material, so both the code and the manufacturer's warranty set a floor.

Material	Minimum slope	Note
Asphalt shingles	2:12	Double underlayment required from 2:12 to 4:12
Wood shingles	3:12
Wood shakes	4:12
Clay or concrete tile	2½:12	Varies by profile, check the manufacturer
Metal panel	Varies	Standing seam goes lower than exposed fastener
Membrane (single ply, BUR)	Below 2:12	What low-slope roofs use

Code IRC 2021 R905 and IBC 2021 Chapter 15 for minimum slopes. Spec exact minimums vary by product; the Asphalt Roofing Manufacturers Association states shingles are intended for slopes no less than 2 inches per foot. Going below the stated minimum generally voids the warranty. Checked August 2026.

Common questions

What is the difference between pitch and slope?

In everyday use they are treated as the same thing, and this calculator does too. Strictly, some traditions define pitch as rise over the full span rather than rise over run, which halves the number. If a figure looks like half what you expected, that is usually why.

Does the multiplier account for hips, valleys and overhangs?

No. It converts footprint area to sloped area for a simple roof. Hips and valleys add cutting waste, and overhangs add area beyond the footprint. Measure the actual footprint including overhangs, then add a waste allowance on top: around 10% for a simple gable, 15% or more where there are several hips and valleys.

Can I put shingles on a flat roof?

No. Below 2:12 asphalt shingles are outside both the code minimum and every manufacturer's warranty, because water can be driven back up under them. A low-slope roof needs a membrane system instead.

How do I measure pitch without climbing on the roof?

From inside the loft, hold a level horizontally against a rafter, mark 12 inches along it, and measure vertically from that mark to the rafter. That vertical distance is the rise in 12. A phone level app against a rafter gives the angle directly.