How this calculator works
Pitch is rise over run, expressed per 12 inches of run. The angle comes from the arctangent of rise ÷ run, and the rafter length from the Pythagorean theorem.
Pitch = Rise ÷ Run × 12 (as “x / 12”)
Angle = atan(Rise ÷ Run)
Multiplier = √(1 + (Rise ÷ Run)²)
Rafter = Horizontal run × Multiplier
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Rise | 6 in |
| Run | 12 in |
| Horizontal rafter run (optional) | 12 ft |
| Result | Value |
|---|---|
| Pitch | 6 / 12 |
| Angle | 26.6 ° |
| Slope | 50 % |
| Pitch multiplier | 1.118 |
| Rafter length (before overhang) | 13.42 ft |
Step by step:
- Pitch = 6 ÷ 12 × 12 = 6 / 12
- Angle = atan(0.5) = 26.57°
- Rafter = 12 ft × 1.118 = 13.42 ft
Roof pitch to angle chart
| Pitch | Angle | Slope | Pitch multiplier |
|---|---|---|---|
| 2/12 | 9.5 ° | 16.7 % | 1.014 |
| 3/12 | 14 ° | 25 % | 1.031 |
| 4/12 | 18.4 ° | 33.3 % | 1.054 |
| 5/12 | 22.6 ° | 41.7 % | 1.083 |
| 6/12 | 26.6 ° | 50 % | 1.118 |
| 7/12 | 30.3 ° | 58.3 % | 1.158 |
| 8/12 | 33.7 ° | 66.7 % | 1.202 |
| 9/12 | 36.9 ° | 75 % | 1.25 |
| 10/12 | 39.8 ° | 83.3 % | 1.302 |
| 12/12 | 45 ° | 100 % | 1.414 |
Common mistakes to avoid
- Measuring rise over a run that is not 12 in, then reading it as x/12.
Frequently asked questions
What angle is a 4/12 pitch?
About 18.4°.
How do I measure pitch?
Hold a level horizontally against the roof, mark 12 in along it, and measure straight up to the roof surface — that is the rise.