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IRC R802
Validated against the standard.
PDF export
Pro-only. Generate professional reports.
$2.99 Pro
Lifetime unlock. No subscriptions.
How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact IRC R802 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
This app figures common rafter length, plumb and seat (birdsmouth) cuts, and overhang from the roof pitch and span, so you can cut a full set of rafters with confidence. It works in feet and inches the way carpenters actually measure.
The geometry
Rafter length = √(rise² + run²)
The run is half the span (to the ridge), and the rise comes from the pitch (for a 6/12 pitch, the roof rises 6 inches for every 12 inches of run). The app adds ridge-board and overhang adjustments.
Worked example
A 24-ft span at 6/12 pitch: run = 12 ft, rise = 6 ft. Rafter line length = √(12² + 6²) = √180 = 13.42 ft before subtracting half the ridge thickness and adding the tail.
Related calculators
Get the surface area with the roof area calculator and check joist sizing with the span table reference.
Rafter FAQ
How is rafter length calculated?
It is the hypotenuse of the rise and run: the square root of rise squared plus run squared, where run is half the building span and rise comes from the pitch.
Does it handle the birdsmouth and overhang?
Yes. It gives the plumb cut, the seat cut for the birdsmouth, and the tail length for the overhang you specify.
Feet and inches?
Yes — input and output in feet-inches-fractions, so you can mark the lumber directly.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Rafter Calculator on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.