Rafter length calculator

Find the sloping rafter line from rise, horizontal run and overhang: length = (run + overhang) × √(1 + (rise/run)²).

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Horizontal run

Rise over that run

More inputs and assumptions

Horizontal overhang

Answer

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Find the sloping rafter line from rise, horizontal run and overhang: length = (run + overhang) × √(1 + (rise/run)²).

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Precision and calculation record

Decimal places in numeric results and text downloads

Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

Saved scenario

Save the current answer, then change an input to compare. This clears when you leave.

Formula and assumptions

This is line length, not a cut list. Ridge thickness, birdsmouth, seat cuts, stock length, structural loads and code checks are excluded.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-26

Using Rafter length calculator

Find the sloping rafter line from rise, horizontal run and overhang: length = (run + overhang) × √(1 + (rise/run)²).

This is line length, not a cut list. Ridge thickness, birdsmouth, seat cuts, stock length, structural loads and code checks are excluded.

Next: Roof pitch calculator

A worked example

Example inputs: horizontal run = 12, rise over that run = 5, horizontal overhang = 0. Sloping length = 13.

Before you use the result

This is line length, not a cut list. Ridge thickness, birdsmouth, seat cuts, stock length, structural loads and code checks are excluded.

Example results for horizontal run

These examples use Rise over that run: 5; Horizontal overhang: 0. They are reference calculations, not recommended settings.

Horizontal run	Sloping length
6	7.81024967591
12	13
24	24.5153013443

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ToolOctopus. “Rafter length calculator.” Updated 2026-09-26. https://tooloctopus.com/rafter-length-calculator.

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