Roof Pitch Calculator
Roof pitch calculator that converts both ways: rise over run, total rise and span, degrees or slope percent into x/12, plus the ridge height for a pitch you pick.
By CalcBuilt Editorial TeamUpdated September 14, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Roof pitch
- 6/12
- 6 in rise over 12 in run
- Nearest standard pitch
- 6/12
- Already a whole pitch, so it carries straight into a framing square, a truss order and the rafter tables
- Roof angle
- 26.6 degrees
- Angle between the rafter and horizontal
- Slope
- 50%
- Rise divided by run x 100
- Pitch factor
- x 1.118
- Multiply the footprint area by this factor to get the roof surface area
- Rafter length per foot of run
- 13.42 in
- Rafter length for every 12 in of horizontal run
Roof Pitch formula
Degrees to pitch: x/12 = tan(angle) x 12 (26.57 degrees = 6/12)
Percent to pitch: x/12 = percent / 100 x 12 (50% = 6/12)
Ridge height = Span / 2 x (x / 12); Rafter = Span / 2 x pitch factor
Pitch = Rise / Run x 12 (written as x/12)
Angle (degrees) = atan(Rise / Run)
Slope % = Rise / Run x 100
Pitch factor = sqrt(1 + (Rise / Run)^2)Worked example
A ridge 5 ft above the top of the walls on a 30 ft wide building: run = 30 / 2 = 15 ft, and rise / run = 5 / 15 = 0.333. The pitch is 0.333 x 12 = 4/12. The angle is atan(0.333) = 18.4 degrees, the slope is 33.3%, and the pitch factor is sqrt(1 + 0.333²) = 1.054. Every foot of run needs 12 x 1.054 = 12.65 in of rafter.
Measured with a level on the roof: a 9 in drop over 12 in is a 9/12 pitch, which is 36.9 degrees, a 75% slope and a factor of 1.25. A house with a 30 x 40 ft roof footprint at 9/12 has 1,200 x 1.25 = 1,500 sq ft of roof surface.
Quick reference
| Pitch | Angle | Slope | Factor |
|---|---|---|---|
| 1/12 | 4.8 deg | 8.3% | 1.003 |
| 2/12 | 9.5 deg | 16.7% | 1.014 |
| 3/12 | 14.0 deg | 25.0% | 1.031 |
| 4/12 | 18.4 deg | 33.3% | 1.054 |
| 5/12 | 22.6 deg | 41.7% | 1.083 |
| 6/12 | 26.6 deg | 50.0% | 1.118 |
| 7/12 | 30.3 deg | 58.3% | 1.158 |
| 8/12 | 33.7 deg | 66.7% | 1.202 |
| 9/12 | 36.9 deg | 75.0% | 1.250 |
| 10/12 | 39.8 deg | 83.3% | 1.302 |
| 11/12 | 42.5 deg | 91.7% | 1.357 |
| 12/12 | 45.0 deg | 100.0% | 1.414 |
How to measure roof pitch
From the roof or a ladder, hold a level horizontally with one end touching the shingles. Mark 12 inches from that end, then measure straight down from the mark to the roof surface. That distance in inches is the pitch over 12. A 6 inch drop is a 6/12 pitch. Doing this on the underside of a rafter in the attic gives the same answer without climbing on the roof.
If you can only measure from the ground, use total rise and span. Measure the building width, and estimate the ridge height above the top of the walls from a photo or by counting siding courses. Half the width is the run, and rise divided by run times 12 is the pitch. On a hip roof the run is still half the width of the building for the main slopes.
Angle, slope and pitch factor
Pitch, angle and slope percent are three ways of describing the same steepness. Roofers use x/12, engineers and saw settings use degrees, and site plans sometimes use percent. Convert pitch to degrees with the arctangent of rise over run: atan(6 / 12) = 26.6 degrees.
The pitch factor is what you multiply the flat footprint area by to get the actual roof surface area. At 6/12 the factor is 1.118, so a 1,000 sq ft footprint has 1,118 sq ft of roof. The same factor gives rafter length per foot of run, which is how rafter tables on a framing square are built.
What this calculator leaves out
The pitch does not say what roofing is allowed. 2021 IRC minimums: asphalt shingles 2/12 (R905.2.2), with double underlayment to 4/12 (R905.1.1). Lapped metal 3/12, or 1/2 in per foot with lap sealant, and standing seam 1/4 in per foot (R905.10.2). Clay and concrete tile 2.5/12 (R905.3.2), wood shakes 4/12 (R905.8.2), roll roofing 1/12 (R905.5.2).
Dry shingles at 6/12 or under are walkable. From 7/12 to 9/12 it is roof jacks and 2x6 toe boards; 10/12 and up means scaffolding or a harness on a ridge anchor. OSHA 29 CFR 1926.501(b)(13) requires fall protection above 6 ft; a harness kit typically costs $100 to $150.
Mistakes that cost money
- Shingling a 1/12 porch roof: IRC R905.2.2 stops asphalt shingles at 2/12; below that use rolled roofing, a membrane or sealed-lap metal.
- Single underlayment on a 3/12 roof: R905.1.1 requires two layers, lapped 19 in, from 2/12 to 4/12, or ice and water over the whole deck.
- Corrugated metal on a 2/12 roof: R905.10.2 sets 3/12 for lapped panels without sealant; standing seam goes to 1/4 in per foot.
- Walking an 8/12 unprotected: 7/12 and up needs roof jacks and 2x6 toe boards, and wet or frosty shingles are unwalkable at any pitch.
- Pricing labor off the footprint: an 8/12 has 20% more area than the footprint and roofers add a steep-slope charge above 7/12.
- A torpedo level on shingle tabs reads a pitch off: use a 2 ft level, or measure a rafter underside in the attic.
Key facts
- A 26.57 degree roof is exactly 6/12 and a 45 degree roof is 12/12, which is why an elevation drawing in degrees has to be converted before a framing square is any use.
- A 12/12 pitch is a 45 degree roof. A 6/12 pitch is 26.6 degrees, not half of 45.
- Pitch factors: 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414.
- The IRC allows asphalt shingles down to a 2/12 pitch, and pitches from 2/12 to 4/12 require two layers of underlayment.
- Most houses have roofs between 4/12 and 9/12. Pitches of 6/12 and under are generally walkable; steeper roofs need roof jacks or scaffolding.
- A 6/12 pitch is the same as a 1:2 ratio or a 50% grade, both of which appear on architectural drawings.
Frequently asked questions
How do I convert degrees to roof pitch?
Take the tangent of the angle and multiply by 12. A 26.57 degree roof is 6/12, 33.69 degrees is 8/12 and 45 degrees is 12/12. Switch this calculator to angle mode and it does the conversion and gives you the nearest whole pitch to build to.
What is a 6/12 pitch in degrees?
26.6 degrees. Divide 6 by 12 to get 0.5, and the arctangent of 0.5 is 26.57 degrees.
Can I walk on my roof?
Dry asphalt at 6/12 or under is walkable in soft-soled shoes. From 7/12 to 9/12 you need roof jacks and toe boards, and 10/12 and up needs scaffolding or a harness tied to a ridge anchor. Wet, frosty or mossy shingles are not walkable at any pitch. OSHA requires fall protection for roofers above 6 ft (29 CFR 1926.501); a harness kit typically costs $100 to $150.
What is the minimum pitch for asphalt shingles?
2/12 under the IRC, with two layers of underlayment required between 2/12 and 4/12. Below 2/12 you need a low slope roofing system such as rolled roofing or a membrane.
What is the pitch factor used for?
Multiply the roof footprint area by the pitch factor to get the sloped surface area for ordering shingles, underlayment or metal panels. Multiply the run by the factor to get rafter length.
Sources and references
- Chapter 9 Roof Assemblies, 2021 IRC (ICC)
- Asphalt Roofing Residential Manual: Design and Application Methods (Asphalt Roofing Manufacturers Association)
- Application of Asphalt Shingles on Slopes Greater than 21:12 technical bulletin (Asphalt Roofing Manufacturers Association)
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