The multiplier is the most useful number for builders: a 6/12 pitch has a 1.118 multiplier, so a 20 ft run becomes 22.4 ft of rafter. It also computes the rafter length directly from the span, so the calculator doubles as a quick layout tool.
Calculator
Formula used
Angle = atan(rise/run) · Rafter length = span × multiplier
How to use the Roof Pitch Calculator
- Measure the rise in inches over a 12-inch horizontal run.
- Type the building span (total width) and the desired overhang.
- Read the pitch, angle, multiplier and rafter length.
- Use the table to compare your roof with common pitches.
About this tool
The Roof Pitch Calculator is built for speed and accuracy: every calculation runs in your browser, so your data never leaves your device and the result updates as you type. It works equally well on a phone, a tablet or a desktop, and it is free to use as many times as you like.
Common use cases
- Cutting rafters before ordering lumber.
- Translating an architect's pitch into a rafter length.
- Verifying a contractor's quote.
- Choosing roofing material (low-slope vs shingle).
Tips for the best result
- Use the latest figures you have - even a small change in rate or term can shift the outcome by hundreds.
- For annual figures, divide by 12 to get a monthly number you can compare with the result.
- Bookmark this page and revisit the calculation as the inputs change.
Frequently asked questions
What is a 6/12 pitch?
Six inches of rise per 12 inches of run, which is about 26.6°. It is a balanced residential pitch that walks well and sheds water without looking steep.
How do I measure my roof?
Climb into the attic and measure the rise from the top of the wall plate to the underside of the ridge, then divide by half the building span to get the rise per foot.
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