Free roofing tool

Rafter length calculator

Find the common rafter length from your run and roof pitch.

Enter your horizontal run and roof pitch to get the common rafter length. Handy for framing a roof, addition, shed, or porch.

Run (feet) Half the building width — the horizontal distance from wall to ridge. Pitch (rise per 12) Inches of rise per 12 inches of run (e.g. 6 for a 6/12 roof).

Common rafter length

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Add overhang length and account for ridge board thickness when cutting.

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How rafter length is figured

The common rafter is the hypotenuse of the run and rise. Length = run × √(1 + (rise/12)²). This gives the line length from wall plate to ridge, before you add the overhang (tail) and subtract half the ridge thickness.

Cost ranges reflect 2026 U.S. installed prices (BLS producer/labor data, Remodeling Cost vs. Value, and manufacturer pricing). Your real price depends on your roof, region, and pro — see the full cost guide.

FAQ

Rafter Length Calculator questions

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How do I calculate rafter length?

Multiply your run by the square root of (1 + (rise/12) squared). For a 12 ft run at 6/12 pitch, that’s about 13.4 ft, before overhang.

What is the run of a rafter?

The run is the horizontal distance the rafter covers — usually half the building’s width, from the outside wall to the center ridge.

Do I add overhang to this length?

Yes. This tool gives the wall-to-ridge line length. Add your desired overhang (tail) and account for the ridge board thickness when marking cuts.

Does this work for hip rafters?

No — this is for common rafters. Hip and valley rafters run diagonally and use a longer factor. A framer or framing calculator handles those.

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