Floor Joist Span Calculator — ft, in to joist size
How to use this calculator
Enter the clear span distance between supports, select joist spacing (12", 16", or 24" on center), wood species, and loading conditions. The calculator determines the minimum joist size needed and shows maximum allowable spans for all common sizes.
Live load: 40 psf for living areas (IRC default), 30 psf for sleeping rooms. Dead load: typically 10-20 psf including subfloor, finishes, and ceiling below. Deflection is checked against L/360 for live load per IRC requirements.
| Joist Size | Actual (in) | Max Span | Deflection | Status |
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Saved Calculations
| Time | Span | Spacing | Species | Load | Recommended | Status |
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How to Size Floor Joists
This floor joist span calculator determines the minimum joist size for a given clear span based on the IRC (International Residential Code) Table R502.3.1. Enter the distance between supports, joist spacing, wood species and grade, and live/dead loads to find the correct joist dimension.
Span Calculation Method
Maximum allowable spans are derived from bending stress (Fb) and modulus of elasticity (E) for each species/grade combination. The calculator checks both bending capacity and L/360 deflection under live load. The more restrictive limit governs. Actual lumber dimensions are used (e.g., 2×10 = 1.5" × 9.25").
Common Floor Loads (IRC)
Living areas: 40 psf live + 10-20 psf dead. Sleeping rooms: 30 psf live. Decks/balconies: 40-60 psf live. Dead load includes subfloor (2-4 psf), ceiling below (5-8 psf), and mechanical/finishes. Always verify with local building codes — some jurisdictions amend IRC tables.
Deflection Limits
IRC requires L/360 for live load deflection and L/240 for total load deflection. This means for a 14-foot span, maximum live load deflection is 14×12/360 = 0.467 inches. Stiffer joists improve floor feel and reduce bounce.
