Free Tool
Free deck joist span calculator
Pick your joist size, spacing, and species to get a reference maximum span for a residential deck. A starting reference, not a stamped design. Free, no signup.
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How far a joist can reach
The span is how far a joist can run between supports before it needs a beam under it. It depends on three things: the size of the joist, how close together they sit, and the species of lumber. Set those and the calculator gives you a reference maximum span.
The numbers here are for a residential deck at a 40 pound live and 10 pound dead load, in the ballpark of the IRC deck tables. Span is measured from the face of one support to the face of the next. Tighter spacing and a stiffer species both let you reach farther, which is why closing the joists from 24 to 16 inches can save you a beam.
Southern Pine carries farther than SPF or Hem-Fir at the same size, so the species toggle matters. If you are near the limit, drop the spacing or step up a joist size rather than pushing the span to the last inch.
This is a reference, not a stamped design. Always build to your local code and the span rating printed on the lumber, and bring in an engineer for anything unusual, heavy, cantilevered, or high off the ground. The tables are a floor for safety, not a target to max out.
Deck Joist Span Calculator questions
How far can a deck joist span?
It depends on the size, spacing, and species. This gives a reference max span for common residential deck lumber, which you confirm against local code.
Does spacing change the span?
Yes. Joists at 16 inches on center reach farther than the same joists at 24, and 12 inch spacing farther still.
Can I build to these numbers?
Treat them as a starting reference, not a design. Build to your local code and the span rating on the lumber, and get an engineer for anything unusual.
Is this really free?
Yes. No email, no signup, and nothing you type leaves your browser.
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Disclaimer: These free tools and templates are provided for general information and estimating purposes only. All results — including quantities, material and cost estimates, load, sizing, and span figures, scores, and any guidance or warnings — are rough approximations that may not fit your specific project, site conditions, local building codes, or current prices. They are not engineering, architectural, structural, legal, financial, or other professional advice, and are not a substitute for a licensed engineer, architect, contractor, or other qualified professional. Always confirm quantities, sizing, and structural, electrical, mechanical, and code requirements with a licensed professional and your local authority having jurisdiction before purchasing materials or performing any work. Werx makes no warranty as to accuracy and accepts no liability for any decision made using these tools. Use at your own risk. See our Terms of Service.