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IRC R502
Validated against the standard.
PDF export
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$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 R502 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 reference does
How far can a joist or rafter span? This app is a quick, offline reference to the IRC span tables for floor joists, ceiling joists, and rafters, by lumber species, grade, size, and spacing — no code book on the jobsite.
What drives a span
- Lumber size — a 2x10 spans much farther than a 2x8.
- Species and grade — Douglas fir-larch and southern pine span farther than spruce-pine-fir at the same size.
- Spacing — 12 in on center spans farther than 16 or 24 in.
- Load — live and dead load, and deflection limit (L/360 for floors).
Worked example
A 2x10 southern pine floor joist at 16 in on center spans on the order of 16 ft under a typical residential live load — while the same joist at 24 in on center spans noticeably less, which is exactly what the table shows.
Related calculators
Apply spans on a deck with the deck calculator and cut roof members with the rafter calculator.
Span table FAQ
Which tables does it use?
The IRC span tables for floor joists (R502), ceiling joists, and rafters (R802), indexed by species, grade, size, spacing, and load.
Why does species matter?
Different species and grades have different strength and stiffness, so a southern pine joist spans farther than a spruce-pine-fir joist of the same size.
Does spacing change the span?
Yes. Tighter spacing (12 in vs 16 in vs 24 in on center) shares the load among more members and increases the allowable span.
Does it work offline?
Yes, the tables are built in and work 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Span Table Reference on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.