How this calculator works
Pitch is how much the roof rises for each 12 inches of horizontal run, written as x/12. Mathematically it is the slope: rise ÷ run. The angle is the arctangent of the slope, and the length of a rafter is the hypotenuse of a right triangle whose base is its horizontal run.
Worked example: a 6 in rise over a 12 in run is a 6/12 pitch: a 26.6° angle and a 50% slope. A rafter with a 15 ft horizontal run is 15 × 1.118 = 16.77 ft long, before overhang.
Tips before you buy
- Measure pitch inside the attic: hold a level horizontally against a rafter, measure 12 in along the level, then measure straight up to the rafter.
- Roofs below 2/12 count as "flat" for roofing purposes and need different materials, such as membrane roofing, not shingles.
- Most residential roofs fall between 4/12 and 9/12.
Tools & materials for this project
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Frequently asked questions
What is a 6/12 roof pitch in degrees?
About 26.6°. A 4/12 pitch is 18.4°, 8/12 is 33.7° and 12/12 is 45°.
How do you convert roof pitch to percent slope?
Divide the rise by the run and multiply by 100. A 6/12 pitch is 6 ÷ 12 = 0.5, which is 50%.
How do I find rafter length?
Multiply the rafter's horizontal run by √(1 + (rise ÷ run)²). For a 15 ft run at 6/12, that is 15 × 1.118 = 16.77 ft.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.