Roof Pitch Calculator
Calculate roof pitch from rise and run, or enter a pitch ratio, angle or slope percentage. See the equivalent angle, slope, pitch multiplier and optional rafter length.
Enter what you know
If you measured the roof with a level, use Rise & Run. If you already know the pitch, choose the matching mode.
Optional: calculate rafter length
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Enter a rise and run to calculate.
How to calculate roof pitch, formulas, examples & FAQs
How to use the roof pitch calculator
- Measure a vertical rise and a horizontal run over the same section of roof.
- Enter both values in the Rise & Run mode, or choose Pitch Ratio, Angle or Slope % if you already know one of those values.
- Read the equivalent roof pitch, angle, slope percentage and pitch multiplier.
- Open the optional rafter-length field when you also need the sloped length for a known horizontal run.
What is roof pitch?
Roof pitch describes how much a roof rises compared with its horizontal run. In common U.S. roofing notation, it is written as rise over 12—for example, 6/12 means the roof rises 6 inches for every 12 inches of horizontal run. The underlying ratio is dimensionless, so the same geometry can be measured in inches, centimetres, feet or metres.
Roof pitch formulas
Worked example: 6/12 roof pitch
Rise/run = 6 ÷ 12 = 0.5.
Pitch = 0.5 × 12 = 6/12.
Angle = arctan(0.5) ≈ 26.57°.
Slope = 0.5 × 100 = 50%.
Pitch multiplier = √(1 + 0.5²) ≈ 1.118.
For a 12 ft horizontal run, rafter length ≈ 12 × 1.118 = 13.42 ft.
How to measure roof pitch
For a simple field measurement, use a level and measure the vertical rise over a known horizontal distance. Keep the rise and run measurements perpendicular: the run is horizontal, while the rafter or roof surface is sloped. A digital level can also report an angle directly, which can be entered in Angle mode.
Roof pitch vs slope percentage
They describe the same geometric steepness in different formats. A 6/12 pitch has a rise/run ratio of 0.5, so its slope is 50%. The angle is about 26.57°. Converting between these representations does not change the roof geometry.
Accuracy and limitations
The formulas are exact for a straight roof plane with a uniform slope. Display values are rounded only after calculation. A real roof may contain several planes, hips, valleys, dormers, overhangs or irregular geometry; calculate each relevant plane separately when estimating surface area.
Rafter length from this calculator is geometric length along the slope for the horizontal run entered. It is not a finished cut length: birdsmouth cuts, ridge details, overhangs, seat cuts, ridge-board deductions and construction tolerances are separate.
Frequently asked questions
What is the angle of a 6/12 roof?
A 6/12 roof has a rise/run ratio of 0.5, which corresponds to about 26.57 degrees.
What is a 4/12 roof in degrees?
A 4/12 pitch has rise/run = 4/12, so the angle is about 18.43 degrees and the slope is 33.33%.
How do I convert roof pitch to slope percentage?
Divide the rise by the run and multiply by 100. For a 6/12 roof: 6 ÷ 12 × 100 = 50%.
How do I calculate rafter length from roof pitch?
For a simple uniform slope, multiply the horizontal run by the pitch multiplier: √(1 + (rise/run)²). The result is the sloped geometric length before construction cuts or overhang details.
Is roof pitch the same as roof angle?
No. Pitch is commonly expressed as a rise/run ratio such as 6/12, while angle is measured in degrees. They describe the same slope and can be converted exactly.
Does this calculator check building codes?
No. Building-code requirements depend on the jurisdiction, roof system, structure, climate and adopted code edition. This tool is a geometry calculator, not a code-compliance checker.