Regulation size hoops, including official NBA size basketball hoops, have a diameter of 18 inches (46cm). The standard diameter of a basketball rim is the same for all senior-level games, including high school and competitive men’s and women’s games.
Children’s hoops that aren’t 10 feet high can sometimes have rims with a smaller ring diameter. As younger children also typically use smaller basketball balls the smaller hoop doesn’t necessarily make it harder to score.