Floor Joist Calculator

Your details

The clear span the joists must bridge from bearing wall to bearing wall.
ft
The dimension of the floor perpendicular to the joists. Used for joist count.
ft
Nominal (labeled) size. Actual dimensions are slightly smaller due to milling.
Species group determines modulus of elasticity (E) and allowable bending stress (Fb).
Higher grades have fewer knots and higher allowable stresses. No. 2 is the most common residential choice.
Distance from center of one joist to center of the next. 16 in is the most common residential standard.
Occupant and furniture weight. IRC requires 40 psf for living areas, 30 psf for bedrooms.
psf
Weight of the floor assembly itself: subfloor, joists, insulation, and finish flooring. Typically 10-15 psf.
psf
Maximum allowable midspan sag expressed as a fraction of span. L/360 is the IRC default for floors.
Retail price per linear foot of the selected joist size. Adjust to local pricing.
USD
Extra material percentage for cuts and waste. 10% is typical for simple rectangular floors.
%
Maximum allowable spanSpan OK
14.88

Smallest of the deflection-governed and bending-governed span limits

Joists required16pcs
Total linear footage246.4
Estimated material cost369.6USD
Span checkPASS - required 14.0 ft is within the 14.9 ft limit
Governing limitBending (strength)
Moment of inertia (I)98.93in⁴
Section modulus (S)21.39in³
Moment of Inertia (in^4)98.93
Section Modulus (in^3)21.39

2 x 10 No. 2 Douglas Fir-Larch spans 14.88 ft - design passes.

  • The maximum allowable span for this joist configuration is 14.88 ft, governed by bending (strength).
  • This floor requires 16 joists total, including 2 rim joists.
  • Estimated lumber cost is $369.60 (with the entered waste allowance).
  • The design passes structural checks for the chosen load, species, and grade combination.

Next stepVerify with your local building department. This calculator uses simplified NDS values and does not substitute for a licensed engineer on permit-required projects.

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