DoItYourself.com stipulates that floor joists are typically spaced about 16 inches apart, measuring from the center of one joist to the center of the next. However, the spacing width also depends on what the specific building code in a given area requires.
Because floor joists bear so much weight, their correct spacing depends on the size and weight of the structure itself. Joists themselves vary in size and material, from wood boards used in smaller buildings to heavy I-shaped metal or metal-reinforced joists used in heavier or taller buildings. When building a home or another building with a raised floor, the best resource for determining proper spacing is the local building code.