Floor Joist Span Calculator
A floor joist behaves like a simply supported beam carrying a uniform load. Its allowable clear span is governed by two checks: bending strength and deflection. The bending span comes from setting the maximum moment equal to the allowable moment (Fb times section modulus). The deflection span comes from holding midspan sag to a fraction of the span, commonly L/360 under live load. This calculator reports both and the controlling (smaller) span. Material design values are species-and-grade specific, so enter Fb, E, and the section properties for your lumber. This is an engineering illustration, not a substitute for code span tables.
Joist span formulas
Line load w = total load (psf) * spacing (ft) ... lb/ft, then /12 to lb/in
Bending span L = sqrt(8 * Fb * S / w_in) ... inches, /12 to feet
Live line load w_l from live load (psf) and spacing
Deflection span L = cube_root(384 * E * I / (5 * x * w_l_in)) ... inches, /12 to feet
Allowable span = min(bending span, deflection span)
The bending check uses the total load; the deflection check uses the live load and the L/x limit. Loads are converted from pounds per square foot to pounds per inch of joist using the tributary spacing.
Joist design context
- Fb and E are species-and-grade design values published by the American Wood Council.
- S and I are section properties of the joist size (for example a nominal 2x10).
- Common residential floors use a 40 psf live load with a roughly 10 psf dead load.
- L/360 for live load is a typical floor deflection limit in residential code.
- Final design must use adopted code span tables or a registered design professional.
Floor joist span: frequently asked questions
How is the maximum joist span calculated?
A floor joist is modeled as a simply supported beam under a uniform load. The bending limit comes from setting the maximum moment (w times L squared over 8) equal to the allowable moment (Fb times section modulus S). Solving for L gives the bending-limited span. A separate deflection limit (often L/360 for live load) usually governs and gives a shorter allowable span.
Where do the strength values come from?
The allowable bending stress Fb and modulus of elasticity E are design values for your specific lumber species and grade, published by the American Wood Council in the National Design Specification supplement. They are not universal constants, so this calculator takes them as inputs. Use the design value for your species, grade, and size as adjusted for the use conditions.
What is the L/360 deflection limit?
Building codes limit how much a floor can sag under live load so it feels solid and finishes do not crack. A common limit is the span divided by 360 for live load deflection. For a uniform load the midspan deflection is 5 w L to the fourth over 384 E I, and setting that equal to L/360 gives the deflection-limited span.
Why does the smaller span control?
Both the bending check and the deflection check must pass. The allowable span is the smaller of the two, because exceeding either one fails the design: bending failure risks collapse, and excess deflection causes a bouncy floor and cracked finishes even if the joist does not break.
Is this a substitute for code span tables?
No. This tool illustrates the engineering relationships using the values you enter. Final design must use the adopted code span tables or a registered design professional, the correct adjusted design values, and the actual dead and live loads for your project. Always verify against the International Residential Code and AWC documents in force locally.
Official sources
- American Wood Council: American Wood Council (NDS design values and span tables).
- International Code Council: International Residential Code (floor framing).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.