Clear span vs joist length — the open air between supports, plus R502.6 bearing

What this diagram shows

A side-view framing diagram of one floor joist running from a sill plate on a foundation wall to a beam on a post. A dimension line labels the clear span — 14 feet 0 inches measured face to face between the supports, the number joist span tables and this calculator use. The joist itself runs longer, because each end must also bear on its support: at least one and a half inches of bearing on wood or steel, and at least 3 inches on masonry or concrete, per IRC R502.6. The bearing zones are highlighted at both ends, and a second dimension shows the actual joist length is roughly the clear span plus both bearings — about 14 feet 3 inches or more — so a 14-foot clear span needs 16-foot stock, not 14-foot. The takeaway: measure the open distance between supports and enter that as the span; buy the next stock length up.