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Roof Pitch Calculator
Type the rise per 12 inches, an angle in degrees, or the ridge height over a building width. The calculator gives the pitch as rise over 12, degrees and percent slope, plus the slope factor used to turn footprint into roof area. A 6/12 roof is 26.6 degrees, a 50% slope.
Updated · Sources re-checked by
6/12 conventional
| Angle | 26.57 degrees |
| Percent slope | 50% |
| Slope factor | 1.118 |
| Rafter length (with overhang) | 14.53 ft |
| Ridge height above the walls | 6 ft |
Multiply a flat footprint area by 1.118 to get the roof surface area.
Add some length to the rafter for the ridge board and the tail cut. Nothing you type is saved or sent anywhere.
How the roof pitch calculator works
Pitch is written as rise over 12: the roof rises that many inches for every 12 inches of horizontal run. Slope = rise ÷ 12. Angle = arctan(slope). Percent slope = slope × 100. Slope factor = √(1 + slope²), which multiplies a horizontal distance to give the distance along the roof surface. From a degree angle, rise per 12 = 12 × tan(angle). From the ridge height over a span, rise per 12 = ridge height ÷ (span ÷ 2) × 12. Rafter length = (half the span + overhang) × slope factor. Ridge height above the walls = half the span × slope.
| Pitch | Degrees | Percent slope | Slope factor |
|---|---|---|---|
| 2/12 | 9.5 | 16.7% | 1.014 |
| 4/12 | 18.4 | 33.3% | 1.054 |
| 6/12 | 26.6 | 50% | 1.118 |
| 8/12 | 33.7 | 66.7% | 1.202 |
| 10/12 | 39.8 | 83.3% | 1.302 |
| 12/12 | 45 | 100% | 1.414 |
| Pitch range | Name | Notes |
|---|---|---|
| Under 4/12 | Low slope | Needs roofing made for low slopes; shingles are limited |
| 4/12 to 9/12 | Conventional | Most homes, walkable with care |
| Over 9/12 | Steep | Needs staging or roof jacks; more labor |
Worked example: a 6/12 roof on a 24 ft wide building with a 1 ft overhang
- Slope: 6 ÷ 12 = 0.5, so a 50% slope.
- Angle: arctan(0.5) = 26.57 degrees.
- Slope factor: √(1 + 0.5²) = 1.118.
- Rafter length: (12 ft + 1 ft) × 1.118 = 14.53 ft along the slope.
- Ridge height above the walls: 12 ft × 0.5 = 6 ft.
Common questions
What does 6/12 pitch mean?
The roof rises 6 inches for every 12 inches of horizontal run. It is a 50% slope and about 26.6 degrees.
How do I measure roof pitch?
Hold a level 12 in out from the roof surface, level it, and measure straight down from the 12 in mark to the roof. That drop is the rise per 12. You can also read the pitch from the framing drawings.
How do I convert degrees to a pitch?
Multiply 12 by the tangent of the angle. A 30 degree roof is about 6.93/12. Type the degrees into this calculator and it does the work.
What is the slope factor used for?
To get from a flat measurement to the sloped surface. Multiply the footprint area of a roof by the slope factor to get the roof surface area, and divide by 100 for roofing squares.
What is the minimum roof pitch for shingles?
Asphalt shingles are commonly installed from 4/12 up, with special underlayment allowed down to 2/12 on many products. Check the shingle maker's instructions and your local code.
How do I find rafter length?
Take half the building width plus any horizontal overhang, and multiply by the slope factor. Add a little extra for the ridge board and the tail cut when you order lumber.
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Sources
- Roof pitch: rise over run, degrees and percent, and the usual names for low, conventional and steep roofs (checked October 28, 2026)
This calculator is an estimate for planning. For rafter cuts and material orders, measure the real roof and check the engineer's or supplier's figures.