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Rafter Length Calculator

Estimate rafter length from roof rise and run, with an optional total length estimate that includes overhang.

Updated April 14, 2026
Free online tool
Planning and research use

Why this page exists

Roof framing math gets easier when rise and run are turned into a rafter length instead of being worked by hand every time. This calculator helps visitors estimate base rafter length from roof rise and run, with an optional simple overhang extension.

Interactive tool

Run the estimate

Enter your numbers and read the result first, then use the sections below to understand what affects the outcome.

Calculator

Rafter length calculator

Estimate rafter length from roof rise and run, with an optional overhang extension.

Result

13.42 ft

Estimated rafter length from right-triangle roof math, with an optional sloped extension for horizontal overhang.

Rafter length13.42 ft

Total length with overhang15.09 ft

Rise used6.00 ft

Run used12.00 ft

Roof angle26.57°

6.00 ft of rise and 12.00 ft of run gives a base rafter length near 13.42 ft.
A horizontal overhang of 1.50 ft adds about 1.68 ft along the roof slope, bringing the total to roughly 15.09 ft.
Use the result as a framing starting point only, because real layout still needs cut allowances, seat details, and local code review.

This is a framing estimate only. It does not fully account for birdsmouth cuts, ridge details, seat cuts, lumber sizing, or local framing requirements.

Planning note

Last updated April 14, 2026. Use this tool to compare scenarios and plan ahead, then confirm important details with the lender, employer, insurer, contractor, or other qualified provider involved in the final decision.

How it works

What the calculator is doing

Enter roof rise and run, and add overhang if you want a simple total sloped length estimate.

The calculator uses right-triangle math to estimate the base rafter length.

If overhang is entered, it converts that horizontal overhang into an approximate sloped extension along the roof line.

Understanding your result

This is a framing estimate only. It does not fully account for birdsmouth cuts, seat cuts, ridge details, or final field layout.

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Examples

Ways people use this tool

Example scenarios help turn a quick estimate into a more useful comparison or planning step.

Estimate a basic rafter length from roof dimensions

A quick hypotenuse estimate can make rough framing plans faster before detailed layout work begins.

See how overhang changes total sloped length

Adding overhang can show how much extra stock length may be needed even when the main span stays the same.

Use it with roof geometry tools

Rafter-length planning often fits naturally beside roof-pitch, roofing, and gable-end area tools.

FAQ

Common questions

How is rafter length calculated here?

The calculator uses the rise and run as the legs of a right triangle and estimates rafter length from the hypotenuse.

How is overhang handled?

If you enter overhang, the calculator converts that horizontal extension into an approximate sloped extension using the same roof slope.

Why is this not a final cut length?

Real framing still needs birdsmouth, ridge, overhang details, and site-specific layout decisions that this simple estimate does not model fully.

Related tools

Keep comparing

Use these related tools to compare nearby scenarios, check a second estimate, or keep narrowing down the right decision.

Home ToolsUpdated April 13, 2026