Floor Joist Span Calculator -

What is a Floor Joist Span?

A floor joist's "span" is the maximum horizontal distance it can extend between two points of support (like a foundation wall and a central beam) without excessive bending or sagging. This calculator uses pre-computed data from standard building code span tables to give you a reliable answer.

The calculation depends on the joist's size (deeper is stronger), the wood's inherent strength (its Fiber Bending or "Fb" value), the spacing between joists, and the acceptable "deflection" (how much it can bend under a load). A safe span is critical for a floor that feels solid and meets safety regulations.

Common Framing Lumber: Species & Use

Species	Strength / Stiffness	Cost	Best For
Douglas Fir-Larch	High	Medium	General framing where strength is needed.
Southern Yellow Pine	Very High	Medium-High	Areas requiring maximum strength, like long spans or heavy loads.
Spruce-Pine-Fir (SPF)	Medium	Low	General purpose framing, cost-effective for standard spans.

How to Use the Floor Joist Calculator

Wood Species & Grade: Select the type of wood you are using. Stronger woods like Southern Pine can span longer distances.
Joist Size: Choose the nominal size of your lumber (e.g., 2x8, 2x10). Taller joists are significantly stronger.
Joist Spacing: Select the on-center spacing for your joists. Closer spacing results in a stronger floor.
Deflection Limit: Choose the stiffness standard. L/360 is standard for living areas, while L/480 is recommended for floors under tile to prevent cracking.
Calculate: Instantly find the maximum distance your selected joist can safely span between supports.

Max Span vs. Joist Size (Douglas Fir #2, 16" O.C.)

Illustrates how span increases dramatically with joist depth

Frequently Asked Questions (FAQ)

What does "Deflection Limit L/360" mean?

"L" stands for the length of the span. A limit of L/360 means the joist is not allowed to bend more than its total span length divided by 360. For a 12-foot (144-inch) span, this is 0.4 inches. This standard prevents a floor from feeling bouncy.

Is this calculator a substitute for an engineer or architect?

Absolutely not. This tool is for estimation and planning purposes only. Always consult local building codes and a qualified structural engineer or architect for any construction project. Span tables do not account for unique loads, holes, notches, or other specific conditions.