What to measure
How to use the roof pitch calculator
Choose what you measured. With a level and a tape, use rise and run: 12 in of run gives the pitch directly, but any run works. From the ground or a plan, use ridge height and building span: the height of the ridge above the top of the wall and the width of the building. If you have a reading from an angle finder or phone app, use roof angle.
The formula
- Pitch (in 12) = 12 × rise ÷ run, or 12 × ridge height ÷ (span ÷ 2), or 12 × tan(angle)
- Angle = arctan(pitch ÷ 12)
- Slope % = pitch ÷ 12 × 100
- Pitch factor = √(1 + (pitch ÷ 12)²)
Worked example: 6 in of rise over 12 in of run
- 12 × 6 ÷ 12 = 6 in 12
- arctan(0.5) = 26.6°, a 50% slope
- √(1 + 0.5²) = 1.118 pitch factor
- √(12² + 6²) = 13.42 in of rafter per foot of run
- A 24 ft span with the ridge 6 ft above the walls gives the same 6 in 12, and a 30° angle reads as 6.93 in 12
Roof pitch chart
| Pitch | Angle | Slope | Pitch factor | Rafter per ft of run |
|---|---|---|---|---|
| 1/12 | 4.8° | 8.3% | 1.003 | 12.04 in |
| 2/12 | 9.5° | 16.7% | 1.014 | 12.17 in |
| 3/12 | 14° | 25% | 1.031 | 12.37 in |
| 4/12 | 18.4° | 33.3% | 1.054 | 12.65 in |
| 5/12 | 22.6° | 41.7% | 1.083 | 13 in |
| 6/12 | 26.6° | 50% | 1.118 | 13.42 in |
| 7/12 | 30.3° | 58.3% | 1.158 | 13.89 in |
| 8/12 | 33.7° | 66.7% | 1.202 | 14.42 in |
| 9/12 | 36.9° | 75% | 1.25 | 15 in |
| 10/12 | 39.8° | 83.3% | 1.302 | 15.62 in |
| 12/12 | 45° | 100% | 1.414 | 16.97 in |
Shingles need at least 2/12, with double underlayment below 4/12 (IRC R905.2.2).
Common mistakes
- Measuring along the roof instead of level. The run must be horizontal. Measuring 12 in along the slope gives a pitch that’s too low.
- Using the full span with the ridge height. The ridge sits over half the span, so the run is span ÷ 2.
- Mixing up pitch and slope percent. A 12/12 roof is 45° and 100% slope; “100% pitch” is not a steeper roof than that.
- Measuring on a sagging rafter. Take the reading near a wall or the ridge, where the rafter is supported, or measure on the gable end.
Frequently asked questions
What does a 6/12 roof pitch mean?
The roof rises 6 inches for every 12 inches it runs level. That's an angle of 26.6° and a 50% slope.
How do I measure roof pitch?
Hold a level against the underside of a rafter or on the roof surface, mark 12 inches along the level, and measure straight up from that mark to the roof. That number is the pitch in 12. From the ground, measure the ridge height above the wall plate and the building span instead.
How do I convert roof pitch to degrees?
Angle = arctan(pitch ÷ 12). 4/12 is 18.4°, 6/12 is 26.6°, 8/12 is 33.7° and 12/12 is 45°.
What is the minimum pitch for asphalt shingles?
2/12. IRC R905.2.2 allows asphalt shingles only on slopes of 2 in 12 or steeper, and requires double underlayment from 2/12 up to 4/12. Flatter roofs need a low-slope membrane.
What is a pitch factor?
The number you multiply a roof's footprint by to get its sloped area, √(1 + (pitch ÷ 12)²). It's 1.118 for a 6/12 roof, so the roof has about 12% more area than its footprint.
Are roof pitch and roof slope the same thing?
They describe the same steepness in different units. Pitch is inches of rise per 12 inches of run (6/12), slope is the same ratio as a percent (50%), and the angle is in degrees (26.6°).
How long is a rafter per foot of run?
√(12² + pitch²) inches. At 6/12 that's 13.42 inches of rafter for every 12 inches of level run.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.