Determine the size of hook needed for fishing based on the approximate size of the fish that you're trying to catch. The larger the fish, the larger the hook size chosen.
The size of the hook is determined by the distance between the shank of the hook and its point. There is no actual exact gap for each size, as each hook is typically sized against others made by the company. Generally, sizes range from 20/0, which is the largest size, to #32, the smallest. The larger the number, the smaller the hook. The hook sizes are even-numbered only until #1, and there is no size 0 hook. Instead of size 0, the numbering system changes to "aughts." For example, a 20/0 is a 20-aught hook.
Select the appropriate size based on manufacturer recommendation or trial and error. Generally, a range of sizes is appropriate for various fish types. For example, when fishing for bass, the appropriate hook size can range from a small #1 to a 6/0. There are also differently designed hooks that affect performance, with styles available such as J and Circle hooks. The multiple designs were created to hook onto the fish in different ways, to ensure that different fish species remain hooked once they've been caught.