Roof pitch calculator
These figures describe the slope you enter. They do not size a rafter or judge whether a roof will carry snow, wind or its own weight — that is a structural matter for your plans or an engineer. Confirm the pitch against the actual roof.
Enter the rise and the run and you get the pitch as a /12 figure, the angle in degrees, the grade as a percentage, the reduced ratio and the rafter multiplier. All of them are the one ratio, rise over run, expressed the way each trade prefers.
Worked example
A roof that rises 6 ft over a 12 ft run is a 6/12 pitch. As an angle that is arctan(6 ÷ 12) ≈ 26.6°, or a 50% grade. The rafter multiplier is √(6² + 12²) ÷ 12 ≈ 1.118, so the rafter over that run is about 13.4 ft before the overhang.
The formula
pitch = (rise ÷ run) × 12 · angle = arctan(rise ÷ run)
Every output is the slope ratio in a different dress: multiply by 12 for pitch, by 100 for grade percent, take the arctangent for the angle, and use Pythagoras over the run for the rafter multiplier. The units cancel in the ratio, which is why pitch is the same whether you measured in feet or metres.
Questions
- What does a roof pitch like 6/12 mean?
- It means the roof rises 6 units for every 12 units of horizontal run — 6 inches of rise per foot. Pitch is always expressed against a run of 12, so a 6/12 and a 3/6 are the same slope written differently. This tool reduces whatever rise and run you enter to the /12 form.
- How do I convert roof pitch to degrees?
- Take the arctangent of rise divided by run. A 6/12 pitch is arctan(6 ÷ 12) = arctan(0.5) ≈ 26.6°. The tool shows the angle, the grade percent and the /12 pitch together, so you can hand over whichever one the next trade wants.
- What is the rafter multiplier?
- It is how much longer the sloping rafter is than the flat run beneath it. For a 6/12 the multiplier is about 1.118, so a rafter over a 12-foot run is roughly 13.4 feet before you add the overhang. It comes straight from the triangle: multiplier = √(rise² + run²) ÷ run.
- Is pitch the same as grade or slope?
- They describe the same steepness in different languages. Pitch is rise per 12 of run, grade is rise ÷ run as a percentage, and the angle is that same ratio in degrees. Roofers talk in /12, earthworks talk in percent, and a protractor reads degrees — this tool gives you all three at once.
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