About the Deck Joist Span Calculator
Properly sizing and spacing deck joists is critical for building a safe, code-compliant deck. Joists that are spaced too far apart or undersized for their span will result in a bouncy, unsafe deck that may not pass inspection. This calculator uses standard IRC span table data for residential decks at 40 PSF live load to recommend the appropriate joist spacing for your configuration.
The maximum allowable span depends on three key factors: joist size (depth), lumber species (which determines strength), and on-center spacing. Larger joists can span farther, stronger species allow longer spans, and closer spacing increases load capacity. Southern Pine is the strongest common framing lumber, followed by Douglas Fir, Hem-Fir, and SPF.
Always verify your local building code requirements, as some jurisdictions have additional requirements beyond the IRC minimums. For elevated decks, hot tub loads, or heavy snow areas, the live load requirement may be higher than the standard 40 PSF, which will reduce allowable spans. When in doubt, go one size up on your joists or reduce spacing for extra safety margin.