Roof Pitch Calculator
Enter the vertical rise and horizontal run to calculate roof pitch ratio, slope percentage, roof angle, and the straight rafter line.
What this calculator does
Roof pitch describes vertical rise relative to horizontal run. Enter rise and run in the same unit to get an X:12 pitch, angle in degrees, slope percentage, slope multiplier, and straight rafter line.
Roof dimensions
Measure rise and run in the same unit. The pitch is normalized to 12 units of run.
The model uses the same rise-to-run ratio as the calculation.
Angle = arctangent(rise ÷ run); rafter line = √(rise² + run²).
The arithmetic uses the exact values shown above. Verify field measurements, product pack sizes, layout constraints, and local requirements before ordering or building.
How to interpret the result
The X:12 result states how many units the roof rises for every 12 horizontal units. The multiplier converts horizontal run to sloped length; it does not include overhangs, cuts, or structural allowances.
How the result is calculated
Slope = rise ÷ run. Pitch per 12 = (rise ÷ run) × 12. Angle = arctan(rise ÷ run). Rafter line = √(rise² + run²).
Worked example
For a 6 inch rise over a 12 inch run, the pitch is 6:12, the slope is 50%, the angle is 26.6°, and the rafter line is about 13.42 inches.
How to use this calculator
- Measure the vertical rise and horizontal run on the same roof plane.
- Select the unit used for both measurements.
- Read the normalized X:12 pitch, angle, percentage, multiplier, and rafter line.
- Add overhangs or construction details only after checking plans and local requirements.
Supported units
Inches, feet, centimeters, and meters. Ratios, degrees, percentages, and multipliers are unit-independent when both measurements use the same unit.
Reference table
| Pitch | Degrees | Slope | Multiplier |
|---|---|---|---|
| 2:12 | 9.5° | 16.7% | 1.0138 |
| 3:12 | 14.0° | 25% | 1.0308 |
| 4:12 | 18.4° | 33.3% | 1.0541 |
| 6:12 | 26.6° | 50% | 1.1180 |
| 9:12 | 36.9° | 75% | 1.2500 |
| 12:12 | 45° | 100% | 1.4142 |
Practical guidance
- Measure rise and run on the same roof plane.
- Use the same unit for both inputs.
- Confirm structural dimensions with plans or a qualified professional.
Assumptions and limitations
The result describes a straight, right-triangle roof section. It does not include overhangs, ridge thickness, birdsmouth cuts, structural loading, or code requirements.
Methodology
The calculator divides rise by run, normalizes that slope to 12 units, uses arctangent for the angle, and applies the Pythagorean theorem to the straight-line rafter distance.
Read the complete calculation methodology and testing policy.
Sources and technical references
References support terminology, units, or practical constraints. They do not imply endorsement or project approval.
- NIST Special Publication 811 — Guide for the Use of the International System of UnitsReference for SI units, symbols, and conversion-factor practice.
- U.S. Department of Energy — Guidelines for Selecting Cool RoofsGovernment guidance illustrating roof-slope terminology and rise-over-run notation.
Frequently asked questions
What does a 6:12 roof pitch mean?
The roof rises 6 units for every 12 units of horizontal run.
Is roof pitch the same as roof angle?
No. Pitch is a rise-to-run ratio; the angle expresses that slope in degrees.
Can I use centimeters or meters?
Yes. The ratio and angle are unit-independent as long as rise and run use the same unit.
Does rafter length include an overhang?
No. The displayed rafter line covers only the entered right triangle.