What size speedo do I need for diving?
31 is an odd size no doubt, but you might actually be lucky. For most male swimmers the rule of thumb is one size larger than you’re waist size for practice, and one size smaller for competition. For diving I would go with one size smaller at all times because you do not want drag of any kind, and of course you don’t want a suit that is going to slip down or off when you enter the water. So go for a 30 or even 28 if it’s comfortable.
