How far apart should bed frame slats be?

For most mattresses, slats should sit no more than about 3 inches apart, and closer is better for foam and hybrid beds. Wide gaps let the mattress sag between the slats, which feels uneven and can void the warranty. Queen and larger frames should also have a centre support leg. If your slats are spaced wider than 3 inches, add a bunkie board or extra slats for even support.