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Roof pitch calculator: degrees, percent, rafter length
Enter the roof pitch in degrees, percent or as a ratio, plus half the building width and the overhang. You get all units, the ridge height and the rafter length.
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How to use it
- Choose the unit and enter the roof pitch.
- Enter half the building width (horizontal run) and the horizontal overhang.
- Read degrees, percent, ratio, tan/cot, ridge height and rafter length.
Last checked: October 2026 · Based on: Trigonometry; minimum roof pitch per manufacturer / roofing trade rules. Found a mistake? Use the form below – we fix confirmed errors and update the date.
Frequently asked questions
How do I convert roof pitch from degrees to percent?
Percent = tan(angle) × 100. Example: 45° = 100 %, 30° ≈ 57.7 %, 10° ≈ 17.6 %.
What does a ratio like 1:2 mean?
1 unit of rise per 2 units of horizontal run – that is 50 % or about 26.6°. The second number is the cotangent of the angle.
How is the rafter length calculated?
Rafter length = horizontal length ÷ cos(pitch). Add the horizontal overhang to the run before dividing.