Roof Pitch Calculator
Convert roof rise and run into pitch, angle, slope percentage, and a geometric rafter factor. You can also start with an angle and convert it to an equivalent rise per 12 inches of horizontal run.
Enter your roof measurements
How the Roof Pitch Calculator Works
All four outputs describe the same right-triangle geometry.
Pitch notation
The calculator normalizes any valid rise/run ratio to an equivalent rise for 12 inches of horizontal run.
Roof angle
Angle from horizontal = arctan(rise ÷ run), converted from radians to degrees.
Slope percentage
Slope percentage = (rise ÷ run) × 100. A 6/12 slope is therefore 50%.
Rafter factor
For pure geometry, factor = √(1 + slope²). Multiply by horizontal run for the theoretical sloped length.
Roof Geometry Is Not Structural Rafter Sizing
This calculator can convert pitch and estimate a geometric straight-line slope length. It does not determine whether a rafter size, species, grade, spacing, or span is structurally adequate. The American Wood Council’s span resources account for structural variables that this geometry calculator does not.
Likewise, the calculated length does not automatically include eave overhang, ridge details, birdsmouth geometry, connection details, or cutting allowances. Use project drawings, applicable codes, manufacturer instructions, and qualified professional guidance where required.
Common Pitch Conversions
These are mathematical conversions, not recommendations that a pitch is appropriate for a particular roof covering.
| Pitch | Angle | Slope | Rafter factor |
|---|---|---|---|
| 2/12 | 9.46° | 16.67% | 1.014 |
| 4/12 | 18.43° | 33.33% | 1.054 |
| 6/12 | 26.57° | 50.00% | 1.118 |
| 8/12 | 33.69° | 66.67% | 1.202 |
| 12/12 | 45.00° | 100.00% | 1.414 |
Roof Pitch Calculation Examples
Each example uses the same geometry as the calculator.
6/12 roof
30° measured angle
4/12 roof
Mistakes to Avoid
Using 12 as the entered run regardless of how you measured
If your actual measurement uses a different horizontal run, enter that value. The calculator will normalize the result to X/12 notation.
Treating slope length as a cut-ready rafter
The geometric hypotenuse does not include overhang, ridge, birdsmouth, connection, or structural design details.
Assuming one pitch works with every roofing product
Minimum slopes and installation requirements depend on the roofing system and manufacturer instructions. Check the exact product specification and applicable code.
Ignoring roof-work fall hazards
Under OSHA 29 CFR 1926.501(b)(13), workers engaged in qualifying residential construction 6 feet or more above lower levels must be protected by conventional fall protection or another measure permitted by the standard. Where the standard’s infeasibility or greater-hazard exception applies, OSHA requires a site-specific fall-protection plan meeting 29 CFR 1926.502(k). This calculator is not a substitute for safe measurement and work practices.
Roof Pitch Calculator FAQ
What does a 6/12 roof pitch mean?
Can I enter a run other than 12 inches?
Can this calculator size structural rafters?
Is the rafter length a finished cut length?
What roof pitch is required for shingles?
Continue Your Construction Estimate
References Used for This Calculator
- American Wood Council — Span Options Calculator for Wood Joists and Rafters: structural context showing that rafter sizing involves variables beyond geometric slope length.
- OSHA — Fall Protection in Residential Construction: safety context for residential roofing and rafter work.
- CalcsDone Methodology: sitewide approach to formulas, assumptions, sources, and limitations.