Floor Joist Span Calculator
Construction & Home ImprovementEstimate the maximum allowable span for a floor joist based on size, spacing, species, load, and deflection limit.
Maximum Span
Largest span at which the joist reaches the chosen deflection limit under the given load, based on stiffness only.
Allowable Deflection at Max Span
0.546
The deflection limit in inches at the calculated maximum span.
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Odeh Ahwal0people find this calculator helpful
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Construction & Home Improvement calculators
Frequently Asked Questions
How is the maximum span calculated?
The span is solved from the standard uniform-load deflection formula, 5 w L to the fourth over 384 E I, set equal to the chosen deflection limit, span over 360 or span over 240. Solving for span gives the largest span at which the joist would just reach that deflection limit under the given load.
Does this replace an official span table?
No. This is a simplified engineering estimate using only modulus of elasticity and deflection, with unadjusted reference E values by species. Published span tables from the American Wood Council or your local building code also factor in bending and shear checks, moisture and duration adjustments, and should be used for actual construction.
What load should I use?
40 pounds per square foot is a common combined live and dead load figure used for residential floor framing examples, but your actual code-required design load depends on the room use and your local building code.
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