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 ?
Floor Load ?
Deflection Limit ?
Check Span
Enter the span you want to check
Enter inputs to see verdict
Results
Joist Diagram
Learn more about floor joists and span calculations
What is the maximum span for a 2x10 floor joist at 16 inch spacing?
It depends on lumber species and floor load. For Southern Yellow Pine No.2 at 40 psf residential load and 16 inch spacing, a 2x10 can span approximately 14 feet 11 inches (4550mm). For Douglas Fir the span is slightly longer due to higher E (modulus of elasticity). The calculator shows exact spans for all four species.
What is L/360 deflection and why does it matter?
L/360 is a deflection limit: the floor joist can sag at most 1/360th of its span under design load. For a 3.6m span that is 10mm of sag. This is the standard for most residential floors. Under tile or marble you need L/480 (stricter) to prevent grout cracks. The calculator checks both and shows which limit controls.
Does joist spacing affect the maximum span?
Yes. Wider spacing means each joist carries more load (larger tributary width), which reduces the maximum safe span. Going from 16 inch to 24 inch spacing can reduce the allowable span by 15-20%. The calculator shows results for all four standard spacings.
FAQ
What is the maximum span for a 2x10 floor joist at 16 inch spacing?
It depends on lumber species and floor load. For Southern Yellow Pine No.2 at 40 psf residential load and 16 inch spacing, a 2x10 can span approximately 14 feet 11 inches (4550mm). For Douglas Fir the span is slightly longer due to higher E (modulus of elasticity). The calculator shows exact spans for all four species.
What is L/360 deflection and why does it matter?
L/360 is a deflection limit: the floor joist can sag at most 1/360th of its span under design load. For a 3.6m span that is 10mm of sag. This is the standard for most residential floors. Under tile or marble you need L/480 (stricter) to prevent grout cracks. The calculator checks both and shows which limit controls.
Does joist spacing affect the maximum span?
Yes. Wider spacing means each joist carries more load (larger tributary width), which reduces the maximum safe span. Going from 16 inch to 24 inch spacing can reduce the allowable span by 15-20%. The calculator shows results for all four standard spacings.