I-Joist Span Calculator

What this I-joist span calculator does

An I-joist is an engineered wood framing member with LVL or solid-wood flanges bonded to an OSB web. The profile looks like a steel I-beam — narrow top and bottom flanges separated by a deeper, thinner web — and that shape is what gives the I-joist its span advantage. The flanges carry bending tension and compression; the web resists shear while adding depth cheaply. Compared to a solid 2x10 or 2x12 of the same depth, an I-joist uses about 60% less wood fiber and reaches spans no solid joist can.

This page is an i joist span calculator built around the structure of a manufacturer's span chart. Pick the TJI series, the on-center spacing, and the load case; the terminal writes out the calculation line by line so you can see exactly which measurement point (live load, dead load, bending moment, deflection limit, bearing reaction) governs the span. The published maximum span for that combination is printed on the green status line, and a target span you enter is checked against it as PASS, MARGINAL (within one inch of the limit), or FAIL.

The reference spans in the calculator and in the wood i beam span tablefurther down the page are educational values modeled on the Weyerhaeuser TJI span chart at 40 psf live / 10 psf dead residential floor load, simple-span, L/480 live-load deflection. Confirm every value against the manufacturer's current published chart and a licensed structural engineer before ordering or building.

Deconstructing the I-joist span calculation

Each line in the terminal above corresponds to one measurement point in the manufacturer's span verification. The table below maps every line to what it measures, the mechanism behind it, and the error / tolerance control that keeps it honest. Treat this as a wood beam span calculator for the engineered-joist product family — same beam theory as solid lumber, different allowable stress table.

The line numbers (L1–L6) appear as Measurement Point tags inside the terminal panel so you can trace each printed value back to the row above.

I-joist anatomy and notation

Wood I-beam span table (TJI series)

The full reference grid for all five TJI series at every common spacing, under the three typical residential floor load cases. Each cell is the maximum span allowed at L/480 live-load deflection. To use the table as a wood i beam span table, scan down the load column for your project, scan across for your spacing, then read the longest span that meets or exceeds your target.

All values educational. Modeled on a Weyerhaeuser TJI span chart at 40/10, 40/20, and 50/20 psf (live/dead) residential floor load, simple-span, L/480. Confirm against the current manufacturer chart before specifying.

I-joist vs solid lumber: side-by-side

Where an I-joist earns its premium and where solid lumber still wins. Use this matrix alongside our solid-lumber floor joist span calculator to decide which product family is right for a given span and budget.

I-joists cost more per linear foot but unlock longer spans at the same depth, deliver an L/480 floor (vs the L/360 typical for solid lumber), and stay flat over time. Solid lumber is still the right answer for short spans, tight budgets, and field conditions where an I-joist web would need heavy notching.

Common I-joist pitfalls and how to avoid them

I-joist installation requirements

Choosing the right I-joist size

The decision flow is simpler than it looks. Start with the longest span on the plan, the spacing you intend to frame at (16" o.c. is the safe default; 19.2" o.c. saves one joist per eight feet), and the load case (40/10 for a normal residential floor, 40/20 for tile, 50/20 for heavy use). Pull the longest target span in feet, then pick the smallest TJI series whose published span in that column exceeds the target by at least one inch.

As a rule of thumb: 9-1/2" depth (TJI 110 / 210) for spans up to about 16 ft; 11-7/8" depth (TJI 230 / 360) for spans 16–24 ft; and 16" depth (TJI 560) for spans 24–32 ft. Above 32 ft, switch to open-web trusses or an LVL floor beam with a shorter joist span between beams. The bigger flange within the same depth steps you up by 2–3 ft of span when the smaller flange is borderline.

Cost trade: a TJI 230 at 16" o.c. for a 22-foot span is usually cheaper than a TJI 360 at 24" o.c. for the same span, because fewer linear feet of joist offset the larger flange. Run both options through the terminal panel above, copy the report, and price them at the lumberyard before committing.

TJI span calculator: one series, one answer

The TJI span calculator is the TJI-specific way to use this page: instead of reading the whole grid at once, you select a single TJI series, a single spacing, and a single load case, and the terminal panel verifies that exact combination line by line — load per joist, bending moment, bending stress, deflection at L/480, and the bearing reaction. It is the "query one cell" complement to the "read every cell" wood i beam span table above.

The two views answer different questions. The span table is for shortlisting: it shows all five TJI series side by side so you can see how each step in depth buys span. The TJI span calculator is for verification: once you have a candidate — say a TJI 230 at 16" o.c. under a 40/10 floor — it prints the full check for that one joist so you can copy the report into a takeoff or a conversation with your supplier. Working the pair together is the intended flow: shortlist on the table, verify on the calculator.

Use the per-series entries below as jump-off points; each names the depth, the flange, and the 40/10 reference span at 16" o.c. that the calculator will confirm:

Reference values modeled on manufacturer-style charts (40 psf live / 10 psf dead, simple span, L/480). Round down to the nearest inch when ordering and confirm against the manufacturer's current published chart — series flanges and allowable loads have been revised over the years.

I-joist span — frequently asked questions