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Est. 1979 | Also: Colville · Kettle Falls | ★ Military & First Responder Discount | Leave a Review
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Floor I-Joist Calculator

Builders Supply · Project Calculators

Floor I-Joist Calculator

Tell us your floor span and joist spacing. We'll show the BCI I-joists that carry it — each tagged with the floor "feel" it delivers, from premium and quiet to code-minimum, so you choose the floor you want.

For reference & budgeting only — not an engineered design.
Final sizes must be confirmed by a licensed engineer before you build or pull a permit. Builders Supply can get your design stamped and supply the material — just ask.

Sizing based on Boise Cascade® published BCI® Residential Floor Span Tables. Builders Supply is a stocking dealer.

How stiff a floor? Three quality levels — same joist, different result
★★★★
Four Star · L/960
Premium, quiet, minimal bounce — the floor a discriminating homeowner notices.
★★★
Three Star · L/480
The industry-standard residential floor — 33% stiffer than code. A solid default.
⚠️ ★★
Code min · L/360
Structurally sound, but a higher risk of bounce. Only where floor feel isn't a concern.

Your floor

ft
in
Clear distance between supports.
Center to center.
Standard — the industry default, 33% stiffer than code. Switch to Premium for a quieter floor, or Code min where bounce isn't a concern.

This sizes

  • Residential floor joists (BCI I-joists), 9½"–16" deep
  • Standard residential load — 40 psf live + 10 psf dead
  • Spacing 12"–32" o.c., with glued-&-nailed sheathing
  • The most restrictive of simple or multiple span

Talk to us about

  • Tile or heavy floors — added dead load needs a tighter check
  • Roof joists — coming soon; ask us in the meantime
  • Cantilevers, point loads, or spans past these tables
  • Anything needing a permit stamp — we route it to our engineer
Want a specific floor feel or an unusual load? Bring your plans to any location and we'll dial it in.

Need it engineered or stamped?

Tile floors, long multi-spans, point loads, or a permit job — Builders Supply will get it engineered and sealed by a licensed engineer, then supply the joists. One stop.

Request a stamp

Spans are read from Boise Cascade's published BCI® Residential Floor Span Tables (Product Data MTP-E7000) for 40 psf live + 10 psf dead residential floors, with 23⁄32" sheathing glued and nailed for composite action, taking the most restrictive of simple or multiple span. Star levels reflect live-load deflection limits (Four Star L/960+, Three Star L/480, Code L/360). Floor "feel" is subjective; deeper joists, a direct-attached ceiling, and a thicker glued subfloor all improve it. This is a material-selection guide — confirm final design against your building code and conditions, or with a licensed engineer for permitted work.

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