Joist Span Calculator
Calculate Maximum Joist Span
Look up maximum joist span based on size and spacing
Formula
Max span from IRC span tables based on joist size, spacing, species, and grade
Frequently Asked Questions
How far can a 2x8 joist span?
A #2 grade SPF 2x8 at 16" on center can span approximately 11 feet 6 inches for a floor (40 psf live load). Douglas Fir can span about 12 feet 6 inches at the same spacing.
Should I use 16" or 24" joist spacing?
16" on center is standard for residential floors and most decks. 24" spacing is sometimes used for roofs or floors with thicker subfloor sheathing. Closer spacing allows longer spans and less floor bounce.
What size joist do I need?
Joist size depends on span length, spacing, and load. For typical residential floors: 2x6 for spans up to 9 ft, 2x8 for up to 12 ft, 2x10 for up to 15 ft, and 2x12 for up to 18 ft (at 16" OC with SPF lumber).
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