The American Wood Council (AWC.org) provides an online calculator that allows you to determine the maximum span of a floor joist. The calculation takes into account the qualities of the wood, installation, load and the environmental conditions.
Whether using a calculator or a table, one of the first criteria to consider is the species of wood, which affects the maximum span. Different types of trees vary by resistance to bending and stiffness. Some projects may be better framed with a wood that bends, while others may require a stiff wood. The joist size also impacts the span, as thicker joists can span a greater distance.
The grade of lumber is another factor to consider. Lower grade woods have more defects, rendering them weaker than more expensive, higher grades. Floor joists are typically made from lumber graded as no. 2, which is not high quality but does not suffer from any great loss in bending strength.
Besides the physical qualities of the wood, the installation and load impact the maximum span of the floor joists. The further apart the joists are spaced, the shorter the span is. Exterior exposure and any incisions shorten the span. The load, including live and dead weight, also affects the span of the joists: the greater the load, the shorter the span.