Roof Pitch Calculator
Enter a rise and a run, or an angle or a percent, and get the roof pitch as a rise in twelve, in degrees and in percent, with the multiplier and the working shown.
Estimates for planning, not professional advice. Read the full notice.
Example: a roof pitch
4 in 12
- Angle 18.43 degrees
- Slope as a percent 33.33 percent
- Multiplier 1.054
Where this pitch sits
- within the 2021 IRC value for asphalt shingles
- at the 2021 IRC double-underlayment range
- outside the 2021 IRC value for steeper slopes
2021 IRC comparison
- The 2021 IRC, section R905.2.2, says “Asphalt shingles shall be used only on roof slopes of 2 units vertical in 12 units horizontal (17-percent slope) or greater. For roof slopes from 2 units vertical in 12 units horizontal … up to 4 units vertical in 12 units horizontal (33-percent slope), double underlayment application is required…”[1]
- The 2021 IRC, section R905.1.2, says “…in areas where there has been a history of ice forming along the eaves … as designated in Table R301.2 … not less than 24 inches (610 mm) inside the exterior wall line … slope equal to or greater than 8 units vertical in 12 … not less than 36 inches (914 mm)…”[1]
Estimate for planning, from what you entered and the product data shown. Check product labels and your supplier before you buy.
Worked example: a roof pitch
Show the working
- Rise divided by run, times twelve 2 ft 6 in ÷ 7 ft 6 in × 12 in = 4 in 12
- Angle: the angle whose tangent is the rise divided by twelve 4 in 12 ÷ 12 in = 18.43 degrees
- Percent: the rise divided by twelve, as a percent 4 in 12 ÷ 12 in = 33.33 percent
- Multiplier: one divided by the cosine of the angle 18.43 degrees = 1.054
Assumptions
The figures behind the example. Sourced figures link to their source below; a default is a starting value to check against your label or supplier.
| Assumption | Value | Details and changes |
|---|---|---|
| Lowest slope for asphalt shingles | 2 in 12[1] | |
| The 2021 IRC, section R905.2.2, says asphalt shingles are used only on slopes of 2 in 12 or greater, and from 2 in 12 up to 4 in 12 double underlayment application is required. | ||
| Slope that adds the along-the-slope ice barrier distance | 8 in 12[1] | |
| The 2021 IRC, section R905.1.2, says an ice barrier extends to a point not less than 24 inches inside the exterior wall line, and on slopes of 8 in 12 or more also not less than 36 inches along the slope from the eave edge. | ||
Find your pitch
- Hold a level against the underside of a rafter, with the bubble centred.
- Measure straight up or down from the level to the rafter at the far end of the level.
- Enter that gap as the rise and the length of the level as the run.
From an angle or a percent
The pitch as a rise in twelve, in degrees, in percent and as a multiplier, starting from an angle, a rise in twelve or a percent
| You have | Rise in twelve | Degrees | Percent | Multiplier |
|---|---|---|---|---|
| 22.5 degrees | 4.97 in 12 | 22.5 degrees | 41.42 percent | 1.082 |
| 5/12 | 5 in 12 | 22.62 degrees | 41.67 percent | 1.083 |
| 12/12 | 12 in 12 | 45 degrees | 100 percent | 1.414 |
| 50 percent | 6 in 12 | 26.57 degrees | 50 percent | 1.118 |
The multiplier is one divided by the cosine of the angle, so it is the same for every roof at that pitch.
Where these pitches sit against the 2021 IRC values
The example pitch is within the 2021 IRC value for asphalt shingles; at the 2021 IRC double-underlayment range; outside the 2021 IRC value for steeper slopes. It is at the 2021 IRC value for the double-underlayment range, quoted below.
- 22.5 degrees: within the 2021 IRC value for asphalt shingles; outside the 2021 IRC double-underlayment range; outside the 2021 IRC value for steeper slopes
- 5/12: within the 2021 IRC value for asphalt shingles; outside the 2021 IRC double-underlayment range; outside the 2021 IRC value for steeper slopes
- 12/12: within the 2021 IRC value for asphalt shingles; outside the 2021 IRC double-underlayment range; within the 2021 IRC value for steeper slopes
- 50 percent: within the 2021 IRC value for asphalt shingles; outside the 2021 IRC double-underlayment range; outside the 2021 IRC value for steeper slopes
2021 IRC slope values
The 2021 IRC wording for asphalt shingle slope and for the steeper-slope ice barrier distance, with the pitch each names
| Item | Pitch the 2021 IRC names |
|---|---|
| Lowest slope for asphalt shingles | 2 in 12The 2021 IRC, section R905.2.2, says asphalt shingles are used only on slopes of 2 in 12 or greater, and from 2 in 12 up to 4 in 12 double underlayment application is required. |
| Upper end of the double-underlayment range | 4 in 12The 2021 IRC, section R905.2.2, says asphalt shingles are used only on slopes of 2 in 12 or greater, and from 2 in 12 up to 4 in 12 double underlayment application is required. |
| Slope that adds the along-the-slope ice barrier distance | 8 in 12The 2021 IRC, section R905.1.2, says an ice barrier extends to a point not less than 24 inches inside the exterior wall line, and on slopes of 8 in 12 or more also not less than 36 inches along the slope from the eave edge. |
Source: [1]
These are model-code values. A pitch is compared with them as within, outside or at the value; the code your area has adopted may differ.
Questions
How do I measure my roof pitch?
Hold a level against a rafter, measure the gap between the level and the rafter at the far end of the level, and enter that gap as the rise and the length of the level as the run. The calculator turns the two into the pitch.
How do I change a pitch into degrees?
Enter the rise in twelve, an angle or a percent. The calculator works out the angle whose tangent is the rise divided by twelve and shows the pitch in all three forms, with the working beside it.
What is the multiplier for?
The multiplier is one divided by the cosine of the angle. Multiplied by the flat area under a plane, it gives the area of the sloped surface, which is how the roofing calculator turns a footprint into roof area.
How does a low pitch compare with the code for asphalt shingles?
The model-code wording for asphalt shingles on low slopes is quoted on this page with its edition and section, and the calculator states whether the pitch you enter is within, outside or at that value. It compares only; your building department decides what applies to your roof.
What is the multiplier when the rise equals the run?
The angle is half a right angle and the multiplier is the square root of two, which the table on this page shows worked out. The multiplier is not a rounded figure copied from another chart.
Local codes and amendments differ; your building department decides.
Sources (1)
-
2021 IRC, Chapter 9 Roof Assemblies, R905.2.2 Slope (opens in new tab), International Code Council, 2021 IRC.
Code or government.
“Asphalt shingles shall be used only on roof slopes of 2 units vertical in 12 units horizontal (17-percent slope) or greater. For roof slopes from 2 units vertical in 12 units horizontal … up to 4 units vertical in 12 units horizontal (33-percent slope), double underlayment application is required…”
“…in areas where there has been a history of ice forming along the eaves … as designated in Table R301.2 … not less than 24 inches (610 mm) inside the exterior wall line … slope equal to or greater than 8 units vertical in 12 … not less than 36 inches (914 mm)…”
Checked November 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 1)
These are summaries of what the sources say, not professional advice. Codes and product data change. Check with your building department and the product label about your own project.
Sources
- [1] International Code Council, 2021 IRC, Chapter 9 Roof Assemblies, R905.2.2 Slope. Checked November 12, 2026. https://codes.iccsafe.org/content/IRC2021P2/chapter-9-roof-assemblies