Floor Joist Span Calculator
Select your lumber species, joist size, and spacing to find the maximum safe span for any floor load. Check any span against the deflection limit and get an instant pass or fail verdict. Nothing uploaded.
Lumber Species
Joist Size
Spacing on Centre ?
Advanced load settings
Floor Load ?
Deflection Limit ?
Check Span
Enter the span you want to check
Enter inputs to see verdict
Results
Full span details
Joist Diagram
Learn more: floor joist span and deflection
Two checks, bending and deflection
The calculator treats each joist as a simply supported beam under an even load. It checks bending stress, using M = wL^2/8 against the allowable stress times the section modulus, and deflection, using 5wL^4/(384EI). A joist must pass both for a Pass verdict, and the maximum span shown is the bending limit only.
Deflection is limited to the span divided by a number, such as 360. Fine Homebuilding's forum discussion of L/360 gives the example of a 12 foot span: 12 feet divided by 360 is 0.4 inches. It also says these limits are normally based on live load only, while the calculator uses the single load you enter for both checks.
A worked example
Take a 2x10 Southern Yellow Pine joist at 16 inch spacing under 40 psf. The load on one joist is 4.44 pounds per inch and the bending limit works out to a 4,460 mm span, about 14 feet 8 inches.
At a 4,500 mm span the bending stress is 814 psi against an allowable 800, so it fails. The deflection ratio there is about L/431, which passes L/360. Bending controls in this case, not deflection.
Species, size and grade
The calculator uses No.2 grade values. For Southern Pine, Hem-Fir and SPF it takes the bending value for each joist size from the Southern Forest Products Association's No.2 design value comparison, which lists Southern Pine at 1,000 psi for a 2x6 falling to 750 psi for a 2x12. Douglas Fir-Larch uses 900 psi throughout. The values for the same species differ by size, and the allowable span grows with the square root of the bending value.
Real designs also apply load duration, repetitive member and moisture adjustments, and building codes have their own span tables. Treat the result as a first check and confirm it against your local code.
FAQ
What is the difference between max span and the span I check?
Max span is the longest span this size and species can carry before the bending check fails. The span you check is tested against both bending and deflection to give a Pass or Fail.
Why does a fail suggest the next joist size?
Going up one nominal size, for example 2x8 to 2x10, adds depth. Stiffness grows with the cube of depth and section modulus with its square, so the next size usually clears both checks.
Does joist spacing matter as much as size?
Yes. Wider spacing gives each joist a wider strip of floor to carry, so the load per joist rises and the allowable span falls.