JOIST SPAN CALCULATOR

Joist Span Calculator — Maximum Floor Joist Span by Lumber Size

Find the minimum joist size needed for your required span. Enter your span distance, joist spacing and live load, and the calculator returns the smallest lumber size (2×6 through 2×14) that meets the span based on IRC prescriptive span tables.

Span & spacing

Wood species

Minimum joist size
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Enter span and spacing to calculate.

Understanding Floor Joist Spans

A joist span is the unsupported horizontal distance a joist must bridge between bearing supports (beams, walls or foundations). The maximum allowable span depends on:

  • Lumber species and grade — Douglas Fir-Larch #2 is the most common reference species
  • Joist size (depth) — deeper joists span farther
  • On-center spacing — closer spacing allows longer spans
  • Live load — the expected weight of people and furniture on the floor
  • Dead load — the weight of the floor structure itself

Live Load Requirements by Application

  • Sleeping rooms (bedrooms): 30 psf live load
  • Living areas, kitchens, dining rooms: 40 psf live load
  • Decks and exterior balconies: 40–60 psf live load
  • Stairs: 40 psf
  • Attic storage: 20 psf
  • Attic habitable space: 30 psf

Always use the appropriate live load for the application. Using 40 psf for a bedroom is conservative and acceptable; 30 psf is the code minimum.

Joist Spacing Options & Trade-offs

  • 12 in o.c.: Stiffest floor feel, least deflection, ideal under tile or stone. Requires more lumber.
  • 16 in o.c.: Standard residential spacing, good balance of stiffness and material cost. Compatible with most subfloor panels.
  • 24 in o.c.: Used with engineered lumber (I-joists, LVL). Reduces material cost but requires thicker subfloor (¾ in minimum).

For ceramic tile or natural stone flooring, 12-inch spacing or engineered I-joists are recommended to minimize deflection and prevent grout cracking.

Frequently Asked Questions

How far can a 2×10 joist span?

A 2×10 Douglas Fir-Larch #2 joist at 16 inches on center can span approximately 16.9 feet for a 40 psf live load. At 12 inches on center, it can span up to 18.6 feet.

Do I need an engineer for floor joist design?

For simple, code-compliant spans using dimensional lumber within the prescriptive tables, an engineer is not required. For unusually long spans, heavy loads, notched joists or engineered lumber, consult a structural engineer.

Can I use engineered lumber to span farther?

Yes. LVL beams, I-joists and PSL can span significantly farther than dimensional lumber for the same depth. Consult the manufacturer's span tables, which are specific to each product.

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