Floor Joist Span Calculator
A 2x10 joist at 16 inches on center spans about 14 feet under a 40 psf live load; tighter spacing or deeper lumber spans farther. This calculator reads IRC-based span tables both ways — span from size, or size from span — plus a deflection check.
Span tables assume ideal conditions: no notches, no big holes, uniform load, dry service. Real framing is rarely ideal.
Planning tool only — not structural advice. Point loads, cantilevers, and anything unusual need a structural engineer, and your local inspector has the final word.
How joist span tables work
Span tables encode three variables: lumber species and grade (stronger wood spans farther), joist depth (deeper is dramatically stiffer — stiffness grows with the cube of depth), and spacing (closer joists each carry less load). The tables assume 40 psf live load plus 10 psf dead load for living areas, and cap deflection at L/360 — the span divided by 360.
That deflection limit is about feel, not strength. A floor at exactly L/360 meets code but can feel bouncy near mid-span, which is why many builders voluntarily design to L/480. The deflection mode above shows you the actual sag number so you can judge for yourself.
Δ = 5wL⁴ ÷ 384EI (must stay under L/360)
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Floor Joist Span Calculator FAQ
How far can a 2x10 floor joist span?
A #2 Southern Yellow Pine 2x10 at 16 inches on center spans 14 feet under a 40 psf live load; at 12 inches on center it reaches 16 feet 2 inches. Douglas fir-larch spans a touch farther. Enter your joist size and spacing in the max-span mode above for the exact table value.
Is a 2x8 or 2x10 better for floor joists?
2x10 spans about 25% farther than 2x8 at the same spacing — 14 feet vs 11 feet 10 inches at 16 inches on center for #2 Southern pine. Use 2x10 when the span demands it; 2x8 is fine and cheaper for shorter runs.
Does joist spacing really change the span?
Yes — a lot. The same #2 Southern pine 2x10 spans 16 feet 2 inches at 12 inches on center, 14 feet at 16 inches, and only 11 feet 5 inches at 24 inches. Tighter spacing is the cheapest way to gain span without deeper lumber.
What does L/360 deflection mean?
It is the code-minimum stiffness limit: under full live load, the joist may sag no more than the span divided by 360. For a 14-foot span that is about 0.47 inches. Floors at L/360 meet code but can feel bouncy — many builders target L/480 for a solid feel. Try the deflection mode above.
Can I drill holes through floor joists for plumbing?
Holes go in the middle third of the joist depth, centered vertically, and no larger than one-third the joist depth — and never in the middle third of the span where bending stress peaks. Notches are even more restricted. When in doubt, ask the inspector before cutting.
Do I need an engineer to size floor joists?
Span tables cover standard cases: uniform loads, normal lumber, simple spans. You need an engineer for point loads (a bathtub, a safe, a grand piano), cantilevers, notched or bored joists, or anything the table does not cover.
What is sistering a joist?
Nailing or bolting a second joist alongside a weak or damaged one to share the load. It is the standard fix for a bouncy floor or a joist that was over-notched — glue plus nails or structural screws, full length, bearing on the same supports.