Circle hooks are typically available in five sizes, including 5/0, 6/0, 7/0, and 8/0 with 5/0 being the smallest and 8/0 the largest commercially sold circle hook. No specified international standard exists for hook sizes; they can vary between manufacturers in just as clothing and shoe sizes do.
Fish hooks are sized backward, however, with the smallest hook being a 32 and the largest commercially available hook typically being a 20/0. Generally the size of the hook is matched with the size of the fish targeted and the type of bait being used. Circle hooks are most commonly used for live and dead bait when angling for large freshwater and ocean tributary fish such as catfish, northern pike and striped bass. A number 32 hook is commonly associated with tiny trout flies and a 20/0 hook is used for deep sea fish up to 800 pounds.