How to Calculate Roof Pitch
Roof pitch determines your rafter length, the roofing materials you can use, and how water and snow behave on the surface. Here's how to measure it, calculate it from any starting point, and use it to estimate materials.
What is roof pitch?
Roof pitch is the ratio of vertical rise to horizontal run, expressed as x:12 in the US. A 6:12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. The same pitch can also be expressed as an angle (26.57°) or a percentage slope (50%).
OSHA's construction fall-protection rule (29 CFR 1926.500) defines two roof categories by pitch:
| OSHA category | x:12 range | Degrees | Fall protection 6 ft+ above a lower level |
|---|---|---|---|
| Low-slope roof | 4:12 or less | 0° – 18.43° | Guardrails, safety nets, personal fall arrest, or warning-line combinations (1926.501(b)(10)) |
| Steep roof | Over 4:12 | Over 18.43° | Guardrails with toeboards, safety nets, or personal fall arrest (1926.501(b)(11)) |
How to measure roof pitch
You can measure from outside on the roof or from inside the attic — the attic method is safer and doesn't require going on the roof.
Method 1 — From the attic (recommended)
- Hold a level horizontally against the underside of a rafter.
- Mark a point exactly 12 inches from where the level touches the rafter.
- Measure straight down from that mark to the rafter surface.
- That vertical measurement in inches is your pitch (e.g. 6 inches = 6:12).
Method 2 — From the roof surface
- Place a level flat on the roof surface, bubble centred.
- Hold one end of the level at the roof surface.
- Measure 12 inches along the level from that end.
- Measure straight down from the 12-inch mark to the roof — that's your rise.
Calculate pitch from rise and run
Example:a roof rises 9 inches over a 12-inch run: 9 ÷ 12 × 12 = 9:12 pitch
If you know the total rise and total run of the roof (not just a 12-inch section):
Example:total rise 8 ft, total run 16 ft: (8 ÷ 16) × 12 = 6:12 pitch
Convert pitch to degrees
| Pitch (x:12) | Degrees | % slope | Pitch factor |
|---|---|---|---|
| 3:12 | 14.0° | 25% | 1.031 |
| 4:12 | 18.4° | 33% | 1.054 |
| 5:12 | 22.6° | 42% | 1.083 |
| 6:12 | 26.6° | 50% | 1.118 |
| 7:12 | 30.3° | 58% | 1.158 |
| 8:12 | 33.7° | 67% | 1.202 |
| 9:12 | 36.9° | 75% | 1.250 |
| 10:12 | 39.8° | 83% | 1.302 |
| 12:12 | 45.0° | 100% | 1.414 |
Calculate rafter length
Once you know the pitch, rafter length follows from the Pythagorean theorem. The rafter is the hypotenuse of the right triangle formed by the rise and run:
Or using the pitch factor as a shortcut:
Example:6:12 pitch, 12 ft run: 12 × 1.118 = 13.42 ft rafter length
Pitch factor — calculating true roof area
The pitch factor converts flat footprint area to sloped surface area — essential for ordering shingles, underlayment, or metal panels:
Example:1,500 sq ft footprint, 6:12 pitch (factor 1.118): 1,500 × 1.118 = 1,677 sq ft of roofing material needed
Calculate your roof pitch instantly
Enter rise and run, degrees, or x:12 ratio to get pitch conversion, rafter length, pitch factor, and true roof area in one step.
Use the Roof Pitch Calculator →FAQ
How do I calculate roof pitch?
Measure the vertical rise in inches over a 12-inch horizontal run. That rise number is your pitch — a 6-inch rise over 12 inches of run is a 6:12 pitch. You can measure from inside the attic by holding a level against a rafter and measuring straight down at the 12-inch mark.
What is the minimum roof pitch for shingles?
Under the 2024 IRC (R905.2.2), asphalt shingles can be used on slopes of 2:12 or greater, with double underlayment required from 2:12 up to 4:12. Clay and concrete tile need 2½:12, metal shingles and wood shingles or shakes 3:12, and slate 4:12. Manufacturer instructions and local amendments can be stricter.
How do I convert roof pitch to degrees?
Use the formula: Degrees = arctan(rise ÷ 12) × (180 ÷ π). Common results: 4:12 = 18.4°, 6:12 = 26.6°, 8:12 = 33.7°, 12:12 = 45°. The roof pitch calculator above handles this conversion instantly.
What is a pitch factor and why does it matter?
The pitch factor is √((rise/12)² + 1). Multiply your roof's flat footprint area by this number to get the true sloped surface area. A 6:12 pitch has a factor of 1.118 — so a 1,000 sq ft footprint needs 1,118 sq ft of shingles or roofing material.
What roof pitch requires fall protection?
Fall protection depends on height, not just pitch. For construction work, OSHA requires guardrails, safety nets or personal fall arrest 6 ft or more above a lower level on steep roofs (over 4:12) and in residential construction at any pitch; low-slope roofs (4:12 or less) also allow warning-line and safety-monitoring systems (29 CFR 1926.501(b)(10), (11), (13)). OSHA has no 12:12 threshold.