"Structural Select" grade 2-by-10 floor joists that are placed 12 inches apart with a live load of 30 pounds per square foot can span up to 21 feet. When the floor joists are placed further apart, such as 16 or 24 inches, or the structural rating of the wood is lower, the span drops significantly.
Floor joists are the support beams that run horizontally underneath flooring. When the flooring is going to be on a second or third level of a house, it is important to know how far the floor joists can be spanned before needing more support. If the floor joists are put in with too little support, the entire floor could collapse. However, if there is excessive support, then the project will be significantly more expensive and redundant.
There are various factors that need to be taken into account when determining the span of floor joists. First, there are different grades of wood that are either harder or softer. The best and most reliable grade for 2-by-10 floor joists is "structural select" grade. Second is the distance between each of the floor joists. Structural select 2-by-10 floor joists that are 12 inches apart can span 21 feet. Those that are 16 inches apart can span 18 feet and 2 inches, while those that are 24 inches apart can only span 14 feet 10 inches.