1. Non-12-inch rise and run
Measured rise = 7.5 in; horizontal run = 18 in.
Pitch = (7.5 ÷ 18) × 12 = 5.00/12
Angle = atan(7.5 ÷ 18) = 22.62°
Slope = (7.5 ÷ 18) × 100 = 41.67%
Rounded to 5/12, 22.62°, and 41.67%.
Roof Pitch Calculator: Rise and Run, Degrees, Slope and Rafter Length
Instant results
Normalized roof pitch
4/12
X/12 means vertical rise for every 12 equal units of horizontal run.
Angle
18.43°
Measured upward from horizontal.
Percent slope
33.33%
Rise ÷ run × 100.
Pitch multiplier
1.054
Converts horizontal run or plan area to sloped length or area.
Unit rafter length
12.65 in
For 12 in of horizontal run.
Actual common-rafter length
—
Geometric length before overhang and cuts.
Sloped roof surface area
—
Excludes waste, overhangs, and complex planes.
Roofing squares
—
1 roofing square = 100 ft². No waste allowance is included.
Worked roof-pitch examples
2. Angle to pitch
Known roof angle = 30° from horizontal.
Ratio = tan(30°) = 0.57735
Pitch = 0.57735 × 12 = 6.93/12
Slope = 0.57735 × 100 = 57.74%
The decimal pitch is retained because 30° is not exactly 7/12.
3. Span, rise, and rafter
Centered gable span = 28 ft; pitch = 6/12.
Run = 28 ÷ 2 = 14 ft
Rise = 14 × (6 ÷ 12) = 7 ft
Rafter = √(14² + 7²) = 15.65 ft
The 15.65 ft result excludes overhang, ridge adjustment, birdsmouth/plumb cuts, and field allowances.
Four ways to measure roof pitch
Prefer ground or attic measurements. Do not climb a roof just to obtain a pitch. If roof access is part of your work, follow applicable fall-protection rules and use trained, properly equipped workers.
From the attic
Hold a level horizontally against the underside of a sloped rafter. From a known run mark—12 in or 30 cm works well—measure vertically to the rafter. Enter that vertical rise and horizontal run.
From a roof or rake edge
Only from a safely accessible position, hold a level horizontally against the roof surface or rake reference. At the known horizontal run mark, measure vertically to the sloped surface. Do not substitute the distance along the shingle or rafter for run.
From total rise and building span
For a centered symmetrical gable, use the vertical rise from wall-top level to ridge and divide the full outside span by two to get one rafter’s horizontal run. Do not halve the span for a shed roof or an offset ridge.
With an angle finder
Zero/calibrate the tool as its maker instructs, then read the roof or rafter angle from horizontal. Enter that reading in Degrees mode. Confirm whether the device reports angle from horizontal or from vertical.
Definitions, formulas, and result interpretation
Pitch and slope
Run is horizontal distance. Rise is vertical height. Rafter length follows the slope.
X/12 = (rise ÷ run) × 12
percent = (rise ÷ run) × 100
Angle conversion
The angle is measured from a horizontal line, not from vertical.
angle = atan(rise ÷ run)
ratio = tan(angle)
Length and area
The multiplier is the sloped length for one unit of horizontal run.
multiplier = √(1 + ratio²)
rafter = run × multiplier
surface area = plan area × multiplier
Common mistakes
- Using sloped rafter length where the formula requires horizontal run.
- Using the full gable span for one common rafter instead of half the span.
- Assuming a calculated geometric rafter length includes overhang, ridge details, cuts, or construction tolerances.
- Treating estimated surface area as an order quantity without adding measured overhangs, waste, starter/ridge materials, and complex roof planes.
Material-selection caution
The calculator describes geometry; it does not approve a roof covering. For example, 2024 IRC §R905.2.2 permits asphalt shingles at 2/12 or greater and points to special underlayment treatment below 4/12. Owens Corning’s Supreme shingle instructions separately specify standard- and low-slope underlayment applications. Verify the code adopted where the building is located and the current instructions for the exact roof covering and underlayment; other systems have different limits.
Roof pitch conversion chart
Values are rounded. Classifications are neutral geometry bands for scanning this table; they are not material approvals or code categories.
| Pitch | Degrees | Percent slope | Multiplier | Descriptive band |
|---|---|---|---|---|
| ¼/12 | 1.19° | 2.08% | 1.000 | Below 2/12 |
| ½/12 | 2.39° | 4.17% | 1.001 | Below 2/12 |
| 1/12 | 4.76° | 8.33% | 1.003 | Below 2/12 |
| 2/12 | 9.46° | 16.67% | 1.014 | 2/12 to <4/12 |
| 3/12 | 14.04° | 25.00% | 1.031 | 2/12 to <4/12 |
| 4/12 | 18.43° | 33.33% | 1.054 | 4/12 to <9/12 |
| 5/12 | 22.62° | 41.67% | 1.083 | 4/12 to <9/12 |
| 6/12 | 26.57° | 50.00% | 1.118 | 4/12 to <9/12 |
| 7/12 | 30.26° | 58.33% | 1.158 | 4/12 to <9/12 |
| 8/12 | 33.69° | 66.67% | 1.202 | 4/12 to <9/12 |
| 9/12 | 36.87° | 75.00% | 1.250 | 9/12 and above |
| 10/12 | 39.81° | 83.33% | 1.302 | 9/12 and above |
| 11/12 | 42.51° | 91.67% | 1.357 | 9/12 and above |
| 12/12 | 45.00° | 100.00% | 1.414 | 9/12 and above |
Roof pitch calculator FAQ
How do you calculate roof pitch from rise and run?
Divide rise by horizontal run to get the slope ratio, then multiply by 12 to express it as X/12. For example, 6 inches of rise over 18 inches of run is (6 ÷ 18) × 12 = 4/12.
How do you convert roof pitch to degrees?
Use angle = arctan(rise ÷ run). For an X/12 pitch, use arctan(X ÷ 12); a 4/12 pitch is about 18.43 degrees.
What do 4/12 and 6/12 equal?
A 4/12 pitch equals about 18.43 degrees, 33.33 percent slope, and a 1.054 multiplier. A 6/12 pitch equals about 26.57 degrees, 50 percent slope, and a 1.118 multiplier.
Are roof pitch and percent slope the same?
They describe the same rise-to-run relationship in different formats. X/12 pitch uses rise per 12 units of horizontal run; percent slope is rise divided by run times 100.
How do you find roof run from building span?
For a symmetrical gable roof whose ridge is centered, horizontal rafter run is half the outside building span. An offset ridge, unequal roof planes, or wall thickness details require each run to be measured separately.
How do you calculate common-rafter length?
Multiply horizontal rafter run by the pitch multiplier, or use the Pythagorean formula √(run² + rise²). The geometric result excludes overhang, ridge-board or ridge-beam adjustment, birdsmouth and plumb cuts, and field allowances.
How does pitch affect roof area?
Multiply the horizontal plan area by the pitch multiplier √(1 + (rise ÷ run)²). This estimates a simple roof surface at one consistent pitch; add overhangs, waste, valleys, hips, and other complex planes separately.
What is the minimum pitch for roofing materials?
There is no universal minimum for every roof covering. As one reference, the 2024 IRC permits asphalt shingles at 2/12 or greater and requires special underlayment treatment below 4/12; always verify the adopted local code and the exact roofing and underlayment manufacturers’ current instructions.
Methodology and review
Last reviewed: 16 July 2026
Editorial owner: Starlight Robotics
Calculation methodology: The calculator uses dimensionless right-triangle trigonometry: ratio = rise/run, angle = arctan(ratio), multiplier = √(1 + ratio²), rafter length = run × multiplier, and sloped area = horizontal plan area × multiplier. The formulas and displayed rounding were checked against the three worked examples above.
References checked: 2024 International Residential Code, Chapter 9; Owens Corning Supreme shingle installation instructions; and OSHA residential fall-protection guidance.
No professional engineering or roofing credential is claimed. This page performs geometry conversions and is not structural design, a cut schedule, or approval for a roofing assembly.