Roof pitch looks like a code the first time you see it — 6/12, 8/12, 12/12 — but it is simply a ratio. Once you know which number is the rise and which is the run, you can turn that ratio into an angle, a rafter length, and a realistic sense of how steep a roof really is. This guide explains roof pitch notation, angle, and rise over run, with a worked example.
For a quick calculation, use the Roof Pitch Calculator. It converts between pitch, angle, and rise and run.
What roof pitch notation means
Roof pitch is written as rise over run, usually with a run of 12. A pitch of 6/12 means the roof rises 6 units for every 12 units of horizontal run. The first number is the vertical rise; the second number is the horizontal run. A larger first number means a steeper roof: 12/12 is much steeper than 4/12.
Do not confuse pitch with slope in degrees, or with a percentage. Pitch is a ratio, angle is measured in degrees, and slope as a percentage is rise divided by run, multiplied by 100. A 6/12 pitch, for example, is a 50% slope, because 6 ÷ 12 = 0.50.
From pitch to angle, and from angle to rafter length
Angle = arctangent (rise ÷ run). For a 6/12 pitch, that is arctangent (6 ÷ 12) = arctangent (0.50) = 26.57 degrees. Rafter length for a simple gable, before any overhang or ridge allowance, follows Pythagoras: rafter length = square root of (run² + rise²).
Worked example: a 6/12 pitch with a 12 ft run
Run = 12 ft, and the pitch is 6/12, so rise = 6 ft. Angle = arctangent (6 ÷ 12) = 26.57 degrees. Rafter length = square root of (12² + 6²) = square root of (144 + 36) = square root of 180 = 13.42 ft.
That 13.42 ft is the sloping length from the wall plate to the ridge for this run, before overhang, ridge board thickness, and any seat cut are allowed for. Use it to understand the geometry, then check the full rafter layout with the Rafter Calculator, which accounts for the details this simple example leaves out.
Common roof pitches at a glance
| Pitch | Rise per 12 in of run | Angle | Feel |
|---|---|---|---|
| 4/12 | 4 in | 18.43 degrees | Low slope |
| 6/12 | 6 in | 26.57 degrees | Moderate slope |
| 8/12 | 8 in | 33.69 degrees | Steep slope |
| 12/12 | 12 in | 45.00 degrees | Very steep slope |
Reading pitch on a real roof
To check a pitch in place, hold a level horizontally from a point on the roof, measure 12 inches along the level, and measure vertically down (or up) from that point to the roof surface. That vertical distance, in inches, is the first number in the pitch: a 6-inch drop over a 12-inch level means 6/12. Take the measurement on a straight section, away from valleys, hips, and sagging boards, and never work on a roof in wet, icy, or windy conditions.
Pitch also affects materials. A steeper roof has more surface area for the same footprint, so it needs more sheathing and more shingles. Once you know the pitch and the roof dimensions, the Roof Sheathing Calculator can help size the sheathing, and the Shingle Calculator can help estimate shingle quantities. Prices vary by region and material, so use those results as estimates to confirm with local suppliers.
Check your numbers
- Roof Pitch Calculator — pitch, angle, rise, and run.
- Rafter Calculator — rafter lengths and layout details.
- Roof Sheathing Calculator — sheathing quantities for a roof.
- Shingle Calculator — shingle quantities for a roof.
Frequently asked questions
What does 6/12 roof pitch mean?
It means the roof rises 6 units for every 12 units of horizontal run. The angle for 6/12 is 26.57 degrees, and the slope is 50%.
How do I convert roof pitch to degrees?
Divide rise by run and take the arctangent. For 6/12, arctangent (0.50) = 26.57 degrees. The Roof Pitch Calculator does this conversion directly.
What is the rafter length for a 6/12 pitch and a 12 ft run?
Using Pythagoras, the square root of (12² + 6²) = 13.42 ft, before overhang and ridge allowances. The Rafter Calculator accounts for those layout details.
Is a higher pitch always better?
Not necessarily. A steeper pitch sheds water and snow more readily, but it also increases roof area, material use, and working difficulty. The right pitch depends on climate, material, design, and local building requirements.