Roof pitch calculator: what is my roof pitch, in degrees and percent?
6/12 pitch
26.6° · 50% slope · pitch factor 1.118 · rafter 13 ft 5 in for 12 ft of run · asphalt shingles allowed from 2/12 (IRC R905.2.2) · for 6 in of rise in 12 in of run
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
6/12 pitch
26.6° · 50% slope · pitch factor 1.118 · rafter 13 ft 5 in for 12 ft of run · asphalt shingles allowed from 2/12 (IRC R905.2.2)
Source: 2021 International Residential Code, Section R905 minimum roof slopes (2021 Seattle Residential Code, chapter 9), read November 8, 2026. Data sheet (PDF)
A roof that rises 6 inches in 12 inches of run has a 6/12 pitch, which is 26.6°, a 50% slope and a pitch factor of 1.118, so 12 ft of horizontal run takes a 13 ft 5 in rafter.
A worked example
A shed roof that rises 7 1/2 in over a 16 in run, with 9 1/2 ft of level run to the rafter needs a 5.63/12 pitch, 25.1° and a 10 ft 5.9 in rafter. 7.5 ÷ 16 is 0.469, which times 12 is 5.63 in of rise per foot. The arctangent of 0.469 is 25.1°, the pitch factor is √(1 + 0.469²) = 1.104, and 9.5 ft × 1.104 is 10.49 ft of rafter before the overhang.
How do I measure roof pitch?
Hold a level flat against a rafter or the roof deck with one end touching it, and mark 12 inches along the level. Measure straight down from that mark to the roof: that distance in inches is the rise, and the pitch is that number over 12. From the attic you can do the same on the underside of a rafter. Enter the rise above with a run of 12, or measure any run you can reach and enter both.
How do I convert a roof angle in degrees to pitch?
Pitch in 12 is 12 times the tangent of the angle. A 30° roof is 12 × 0.577 = 6.93, close to a 7/12, and a 45° roof is exactly 12/12. Going the other way, the angle is the arctangent of rise over run, which is what the calculator does. The chart below lists every pitch from 1/12 to 12/12 with its angle, so you can find the nearest one by eye.
What is the minimum roof pitch for shingles, tile and metal?
The 2021 IRC sets the floor by covering. Asphalt shingles need 2/12, and from 2/12 up to 4/12 they go over a double layer of underlayment. Clay and concrete tile need 2 1/2/12, also doubled below 4/12. Metal roof shingles, wood shingles and shakes need 3/12, and slate 4/12. Lapped metal panels need 3/12, or 1/2/12 with lap sealant; standing seam goes down to 1/4/12, like modified bitumen and single-ply membranes. Pick your covering above and the calculator checks your pitch against it.
What else it is called
Roof pitch is also called roof slope or roof angle. Builders write it as rise in 12, like 4/12 or 6/12, said “four twelve” or “six in twelve”; engineers use degrees or percent. All three describe the same triangle, so the table below converts between them. The roof covering you pick sets the lowest pitch the code allows: the calculator flags a pitch under that minimum, and the band below 4/12 where shingles and tile need double underlayment.
Roof pitch chart
| Pitch | Angle | Slope | Pitch factor |
|---|---|---|---|
| 1/12 | 4.76° | 8.3% | 1.003 |
| 2/12 | 9.46° | 16.7% | 1.014 |
| 3/12 | 14.04° | 25.0% | 1.031 |
| 4/12 | 18.43° | 33.3% | 1.054 |
| 5/12 | 22.62° | 41.7% | 1.083 |
| 6/12 | 26.57° | 50.0% | 1.118 |
| 7/12 | 30.26° | 58.3% | 1.158 |
| 8/12 | 33.69° | 66.7% | 1.202 |
| 9/12 | 36.87° | 75.0% | 1.250 |
| 10/12 | 39.81° | 83.3% | 1.302 |
| 11/12 | 42.51° | 91.7% | 1.357 |
| 12/12 | 45.00° | 100.0% | 1.414 |
Angle is the arctangent of rise over run; slope is rise over run as a percentage; the pitch factor, √(rise² + 12²) ÷ 12, turns a footprint into roof surface.
Where the numbers come from
- Pitch is rise over run scaled to 12 inches of run: rise in × 12 ÷ run in. The angle is the arctangent of rise ÷ run, the percent slope is rise ÷ run × 100, and the pitch factor is √(1 + (rise ÷ run)²), the length of the sloped side for each unit of level run. All of it is trigonometry on the two numbers you measure.
- Rafter length is the level run times the pitch factor, measured along the top of the rafter from the ridge line to the outside of the wall. Overhang and the ridge board thickness are added or taken off by hand.
- Minimum pitch by roof covering: 2021 International Residential Code, Section R905, read in the 2021 Seattle Residential Code and checked against the 2021 IRC as adopted in New Jersey. Asphalt shingles «only on roof slopes of 2 units vertical in 12 units horizontal» or greater, with double underlayment from 2/12 up to 4/12 (R905.2.2); clay and concrete tile 2 1/2/12, double underlayment up to 4/12 (R905.3.2); metal roof shingles 3/12 (R905.4.2); mineral-surfaced roll roofing 1/12 (R905.5.2); slate 4/12 (R905.6.2); wood shingles and shakes 3/12 (R905.7.2, R905.8.2); lapped metal panels 3/12 without lap sealant and 1/2/12 with it, standing seam 1/4/12 (R905.10.2); modified bitumen and single-ply membranes 1/4/12 (R905.11.1 to R905.13.1). Read November 8, 2026. Read the code chapter (PDF)
The rafter length is the line length from ridge to wall: add the overhang you want past the wall, and take off half the thickness of the ridge board.
By Javier Miralles · Last updated . How this site is made.
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