How is roof pitch calculated?
Roof pitch is the vertical rise divided by the horizontal run, stated per 12 inches of run. A roof that rises 6″ for every 12″ it runs is a 6:12 pitch — about 26.6° or a 50% grade. Measure any rise and run in the same units and this calculator converts between all three formats.
Run is measured from the outside wall to a point directly beneath the ridge, which on a simple gable roof is half the building span, not the whole width. Getting that wrong is the single most common mistake in roof estimating: using the full span halves the pitch you calculate.
The formula
Worked example — a roof rises 6″ over a 12″ run:
What the slope multiplier is for
A roof always has more surface than the ground it covers. If you measure a house at 40 ft × 30 ft, that 1,200 sq ft footprint is not how much roofing you need to buy — the sloped surface above it is larger, and the steeper the roof, the larger it gets.
The slope multiplier is the conversion factor. At 6:12 it is 1.1180, so that same footprint needs 1,200 × 1.1180 = 1,342 sq ft of roof covering before waste, hips and valleys. The same number turns a run into a rafter length: a 15 ft run on a 6:12 roof gives a 15 × 1.1180 = 16.77 ft rafter line, measured from the outside wall to the ridge before you add any overhang.
Common roof pitches
| Pitch | Angle | Typical use |
|---|---|---|
| 1:12–2:12 | 4.8°–9.5° | Low slope — membrane or rolled roofing only, not shingles |
| 3:12 | 14.0° | Porches, additions — shingles allowed with doubled underlayment |
| 4:12–6:12 | 18.4°–26.6° | Most common on US homes; walkable with care |
| 7:12–9:12 | 30.3°–36.9° | Steep — roof jacks and staging required to work on it |
| 10:12–12:12 | 39.8°–45° | Very steep — A-frames, Victorians; specialist installation |
Roof pitch multiplier table
The slope multiplier — also called the roof pitch factor — converts a flat, plan-view measurement into the real distance along the roof. Multiply the run by it to get a rafter length, or multiply the building footprint area by it to get the actual roof area you have to cover.
| Pitch | Angle | Grade | Multiplier |
|---|---|---|---|
| 1:12 | 4.76° | 8.3% | 1.0035 |
| 2:12 | 9.46° | 16.7% | 1.0138 |
| 3:12 | 14.04° | 25% | 1.0308 |
| 4:12 | 18.43° | 33.3% | 1.0541 |
| 5:12 | 22.62° | 41.7% | 1.0833 |
| 6:12 | 26.57° | 50% | 1.1180 |
| 7:12 | 30.26° | 58.3% | 1.1577 |
| 8:12 | 33.69° | 66.7% | 1.2019 |
| 9:12 | 36.87° | 75% | 1.2500 |
| 10:12 | 39.81° | 83.3% | 1.3017 |
| 11:12 | 42.51° | 91.7% | 1.3566 |
| 12:12 | 45.00° | 100% | 1.4142 |
Multiplier = √((rise ÷ run)² + 1), rounded to four decimals.
Frequently asked questions
What roof pitch is most common?
Most US homes are between 4:12 and 6:12. It sheds water well, shingles install normally, and it's still safe to walk with proper footwear and a dry surface.
What's the minimum pitch for shingles?
Building code generally allows asphalt shingles down to 2:12, but between 2:12 and 4:12 you need doubled underlayment or an ice-and-water membrane. Below 2:12, use a membrane or rolled roofing instead. Always check the shingle manufacturer’s instructions and your local code, since both can be stricter.
How do I measure pitch without getting on the roof?
In the attic, hold a 12″ level horizontally against the underside of a rafter and measure straight down from its free end to the rafter. That measurement in inches is your rise per 12″ of run — enter it above with a run of 12.
Is pitch the same as slope?
In everyday use, yes. Strictly, slope is rise over run (a ratio or percent), while pitch was historically rise over the full span. Roofers today say "pitch" for the X:12 ratio — that's what this calculator shows.
Can I use metric measurements?
Yes — switch the units to centimetres or metres. The ratio itself is dimensionless, so a roof rising 15.24 cm over 30.48 cm is still a 6:12 pitch. The angle, grade and multiplier are identical either way; only the rafter length is reported in the unit you picked.