How to use the roof pitch calculator
- Choose whether you know the rise and run, or the angle.
- Enter rise and run in the same units. Any run works; the calculator scales it to 12.
- Read the pitch, angle, slope percentage and pitch factor.
Where it is safe, measure pitch from inside the attic on a rafter rather than on the roof surface, as Decoratoradvice recommends.
Roof pitch formulas
Pitch = rise ÷ run × 12, written as "x in 12" or x/12
Angle = arctan(rise ÷ run)
Slope % = rise ÷ run × 100
Pitch factor = √(1 + (rise ÷ run)²)
Worked examples
A rise of 6 in over a run of 12 in:
Pitch: 6 in 12
Angle: arctan(0.5) = 26.6°
Slope: 50%
Pitch factor: 1.118
From an angle of 30°:
Pitch: tan(30°) × 12 = 6.93 in 12
Slope: 57.7%
Common pitches
| Pitch | Angle | Slope % |
|---|---|---|
| 3/12 | 14.0° | 25% |
| 4/12 | 18.4° | 33.3% |
| 5/12 | 22.6° | 41.7% |
| 6/12 | 26.6° | 50% |
| 8/12 | 33.7° | 66.7% |
| 10/12 | 39.8° | 83.3% |
| 12/12 | 45.0° | 100% |
How to measure pitch
From the attic or a gable end, hold a level horizontally against the underside of a rafter. Mark 12 in along the level, then measure straight up from that mark to the rafter. That measurement is the rise in inches: the pitch is that number in 12. You can also measure any convenient run and enter both numbers.
Why pitch matters
- Roof area: steeper roofs have more surface for the same footprint. The roofing calculator uses the pitch factor.
- Rafters: rafter length depends on pitch; see the rafter length calculator.
- Materials: shingle and metal panel manufacturers set a minimum pitch for each product. Check it before choosing a roof covering; the metal roofing calculator covers panels.
Common mistakes
- Measuring along the slope. Run is horizontal; measuring along the rafter gives the wrong ratio.
- Confusing pitch and angle. A 12/12 pitch is 45°, not 12°.
- Mixing units for rise and run.
Frequently asked questions
What is a 4/12 roof pitch in degrees?
18.4°. The roof rises 4 in for every 12 in of run.
How do I convert roof pitch to degrees?
Divide the rise by 12 and take the arctangent. For 8/12, arctan(0.667) = 33.7°.
What is the difference between pitch and slope?
They are often used interchangeably. Here, pitch means rise per 12 in of run, and slope means the same ratio as a percentage.
What is a pitch factor used for?
Multiply a horizontal measurement by the pitch factor to get the length along the roof. It turns footprint area into roof area.
Is a steeper roof better?
Steeper roofs shed water and snow faster but cost more to build and are harder to work on. The right pitch depends on design, climate and roofing material.
Can I use this for stairs?
The angle maths is the same. For riser and tread counts, use the stair calculator.
Key takeaways
- Pitch = rise per 12 in of run.
- Angle = arctan(rise ÷ run); a 6/12 roof is 26.6°.
- Pitch factor = √(1 + (rise ÷ run)²).
- Measure run horizontally, not along the rafter.