2x10 southern pine
Typical deck joist planning lookup.
15.3 ftSource-indicated max spandeck joist span calculator
Deck Joist Span Calculator
Deck joist span depends on species, grade, size, spacing, loading, wet-service conditions, cantilever, and the adopted local code.
Inputs
Results
Immediate estimateSource-indicated max span15.3 ftProposed span is within this limited lookup.
Proposed span12 ft
Spacing16 in o.c.
Joist size2x10
ResultWithin lookup
Review the limitations before using this result for planning.
This MVP lookup is limited to selected species, sizes, spacings, and simple loading assumptions. Do not extrapolate.
Diagram
Deck joist span
The proposed clear span is compared with a limited source-backed lookup row.
- Beam
- Ledger
- Clear span
How this calculator works
Select species group, joist size, spacing, and proposed span to compare against a limited planning lookup.
Lookup only
This MVP does not interpolate or extrapolate. If a row is unsupported, it returns no recommendation-like span.
Cautious publishing
The page is useful for early planning and SEO, but it keeps limitations above the fold and in the calculator warnings.
Deeper design work
A production technical version should add grade, wet-service factors, cantilevers, beam reactions, and edition-specific tables.
Lookup
match species group + size + spacing to supported rowComparison
proposed clear span <= source-indicated spanAssumptions
- Use: Deck joist planning under selected simple residential assumptions.
- Limits: Ledger, beam, fastener, cantilever, wet-service, grade, and local amendments are not checked.
Worked examples
Unsupported 12-inch hem-fir
Shows unsupported-row behavior.
No supported rowSource-indicated max spanFAQ
Can I use this for permit drawings?
No. Use adopted local code tables or a qualified designer for permit documents.
Why are some combinations unsupported?
The MVP intentionally includes a small lookup surface and refuses to extrapolate beyond it.
Does this check cantilevers?
No. Cantilevers and overhangs need separate checks.
Sources and review
- 2021 International Residential Code, International Code Council, 2021. Accessed 2026-07-16.
- Wood Handbook: Wood as an Engineering Material, USDA Forest Products Laboratory, Revised 2021. Accessed 2026-07-16.
Reviewed 2026-07-16. Planning estimates only; confirm project-specific requirements.