How far apart are floor joists?
Floor joists are most commonly spaced 16 inches on-center (o.c.); 12, 19.2, and 24 in o.c. are also used. There's no single required spacing — the IRC sets the allowable span for a given size, species/grade, and load in its floor-joist span tables (§R502.3), so the spacing you can use depends on how far the joists have to carry.
Source: 2021 IRC §R502.3 (floor-joist spans) · Verify against your AHJ and the adopted code edition.
On-center (o.c.) spacing is measured from the center of one joist to the center of the next. In practice, floor joists land on a few standard spacings:
- 16 in o.c. — the common default; lines up with 4- and 8-foot sheet goods.
- 12 in o.c. — tighter spacing for longer spans or heavier loads.
- 19.2 in o.c. — the “five per 8 feet” layout, used to stretch material.
- 24 in o.c. — fewer joists, but needs deeper/stronger members for the span.
There isn’t one code-mandated number. What governs is the allowable span: for a given joist size, species and grade, spacing, and load, the IRC gives the longest distance the joist may clear in its floor-joist span tables under §R502.3 (we cite the section and link out rather than reproduce the tables). Pick a spacing, then confirm the size and span against those tables and your local amendments.
Work it out: for the on-center layout itself — how many members across a run and where each lands — the stud-spacing reference does the same center-to-center count for any framing at 16 or 24 in o.c.
Sources
- ICC Digital Codes — 2021 IRC — §R502.3 (allowable floor-joist spans)