How to Calculate Roof Pitch
Published 2026-09-04
Roof pitch is the rise over 12 inches of horizontal run. Hold a level against the roof, mark 12 inches along it, and measure straight down to the roof surface. A 6-inch drop is a 6/12 pitch, which is 26.6 degrees.
Every roofing quantity depends on pitch. It sets how much bigger the roof is than the building underneath it, which shingles are allowed, and whether the job needs staging or just a ladder. It takes two minutes to measure.
What pitch actually means
Roofers state slope this way as a matter of convention — the National Roofing Contractors Association uses the same rise-over-12 notation throughout its technical manuals.
Pitch is written as a ratio — 4/12, 6/12, 9/12 — and it always means inches of vertical rise per 12 inches of horizontal run. A 6/12 roof climbs 6 inches for every foot you move sideways. The second number is always 12, so “a 6 pitch” and “6/12” are the same thing.
Measuring it with a level
- Hold a 2-foot level horizontally against the roof surface, one end touching.
- Mark 12 inches along the level from the touching end.
- Measure straight down from that mark to the roof.
- That measurement, in inches, is your rise. The pitch is rise/12.
You can do the same thing in the attic against the underside of a rafter, which is safer and gives the same answer. On a finished ceiling, measure against a gable-end rafter where it is exposed.
Change any measurement to update your estimate.
- Shingle bundles
- 37 bundles
- Slope factor
- ×1.118
- Footprint
- 1,000 ft²
Pitch, degrees, and slope factor
degrees = arctan(rise ÷ 12)
slope factor = √(rise² + 144) ÷ 12The slope factor is the number you actually use: multiply the building footprint by it to get true roof area — that is exactly what the roofing calculator and the roof pitch calculator do with the numbers you measure.
| Pitch | Degrees | Slope factor | Walkable? |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | Yes — but too flat for most shingles |
| 3/12 | 14.0° | 1.031 | Yes |
| 4/12 | 18.4° | 1.054 | Yes |
| 5/12 | 22.6° | 1.083 | Yes |
| 6/12 | 26.6° | 1.118 | Usually |
| 7/12 | 30.3° | 1.158 | With care |
| 8/12 | 33.7° | 1.202 | Roof jacks needed |
| 9/12 | 36.9° | 1.250 | No |
| 10/12 | 39.8° | 1.302 | No |
| 12/12 | 45.0° | 1.414 | No |
Why the slope factor matters
A 40 by 25 foot building has a 1,000 ft² footprint. The roof over it is not 1,000 ft²:
| Pitch | Roof area | Squares |
|---|---|---|
| 4/12 | 1,054 ft² | 10.5 |
| 6/12 | 1,118 ft² | 11.2 |
| 9/12 | 1,250 ft² | 12.5 |
| 12/12 | 1,414 ft² | 14.1 |
Order shingles off the footprint for a 9/12 roof and you are 25% short — five squares missing on a twelve-square job.
Low slope changes the material
Below 4/12, asphalt shingles need a doubled underlayment, and most manufacturers void the warranty below 2/12 entirely. That is where roofs move to rolled roofing, modified bitumen or a membrane. The International Residential Code sets the minimum slope for asphalt shingles at 2:12 with the extra underlayment; check your local amendment before ordering.
The one measurement people get wrong
Pitch is measured on the horizontal run, not along the roof surface. If you run your tape up the slope for 12 inches and measure the rise from there, every number comes out low. Keep the level level.