Roof Pitch Calculator

The formula

Coverage and yield assumptions

Rise and run can be in any unit as long as both use the same one. The multiplier converts horizontal (plan) area into sloped surface area, the factor used by NRCA and roofers to estimate materials.

How to measure pitch

From inside the attic, hold a level horizontally against a rafter, mark 12 in along it, and measure vertically from that mark up to the underside of the rafter: that measurement is the rise. On the roof, place a 2 ft level with one end touching the shingles, level it, and measure from the 12 in mark down to the surface. Smartphone apps that read the angle of a rafter also work; convert the angle with tan(angle) × 12.

Pitch categories

Below 2/12 is low slope and needs membrane roofing such as TPO, EPDM or modified bitumen. 2/12 to 4/12 can take shingles with extra underlayment. 4/12 to 9/12 is conventional and walkable for most roofers. Above 9/12 is steep and often costs more to install because it needs roof jacks and harnesses.

Common pitches and angles

• 3/12 = 14.0°
• 4/12 = 18.4°
• 5/12 = 22.6°
• 6/12 = 26.6°
• 8/12 = 33.7°
• 10/12 = 39.8°
• 12/12 = 45°

Rafter length

The same multiplier gives rafter length: multiply the horizontal run of the rafter (half the building span, plus overhang) by the multiplier. For a 30 ft wide building with a 6/12 pitch, the run is 15 ft and the rafter is about 16.8 ft before overhang and ridge adjustments.

Common mistakes to avoid

• Mixing units, such as rise in inches and run in feet.
• Measuring from the fascia board, which may not follow the rafter.
• Treating degrees and pitch as the same number.
• Assuming the whole roof has one pitch when additions differ.
• Using pitch alone to choose materials without checking the maker's minimum slope.

Why pitch matters for cost

Steeper roofs have more surface area for the same footprint, so they need more shingles, underlayment and labor. A 12/12 roof has about 41% more area than a flat projection of the same house, while a 4/12 roof adds only about 5%. Steep roofs also need roof jacks, harnesses and slower work, and some contractors add a steep-slope charge above about 8/12. Low-slope roofs drain slowly, so they rely on membranes and careful flashing instead of overlapping shingles.

Pitch and snow, wind and solar

Steeper pitches shed snow and rain faster, which is why they are common in heavy snow regions. Very low pitches can hold snow loads and need stronger framing. For rooftop solar panels, a pitch close to your latitude in degrees often maximizes yearly output, but most installers mount flush on any common pitch. In high-wind areas, hip roofs with moderate pitches between about 4/12 and 6/12 tend to perform well.

Converting between formats

To convert degrees to pitch, multiply the tangent of the angle by 12: tan(30°) × 12 = 6.93, so 30° is about a 7/12 pitch. To convert percent slope to pitch, multiply by 0.12: a 25% slope is a 3/12 pitch. Builders sometimes describe pitch as a fraction of the full span, an older convention; a "1/4 pitch" means a rise of one quarter of the span, which equals 6/12.

Waste factor

Pitch is a measurement, not a material, so this page has no waste factor. Use the roofing calculator to apply waste to shingles once you know the pitch.

Imperial and metric

Switch the toggle above the inputs to enter in ↔ cm. The page converts metric entries to imperial before calculating, using NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm), and shows key results in both systems.

Worked examples

Measured 6 in over 12 in

Inputs: rise 6 in; run 12 in.

Result: Your roof pitch is 6/12 (26.6°).

• Slope as percent: 50%
• Area multiplier: 1.118

Shed roof 2 in over 12 in

Inputs: rise 2 in; run 12 in.

Result: Your roof pitch is 2/12 (9.5°).

• Slope as percent: 16.7%
• Area multiplier: 1.014

Rafter measured 54 in rise over 72 in run

Inputs: rise 54 in; run 72 in.

Result: Your roof pitch is 9/12 (36.9°).

• Slope as percent: 75%
• Area multiplier: 1.25

Quick reference for common sizes

The table repeats the first worked example at smaller and larger sizes, keeping every other setting the same, so you can see how the roof pitch calculator result scales.

Limitations

Results are geometric. Material suitability depends on the manufacturer's minimum slope, local climate and code.