The best way is to measure the head of the recipient with a tape measure. This will give you an accurate circumference so you can choose the best hat size to make for the man in your life. A lot of times for an adult male that will be an adult large, around a 22" circumference, but of course men's heads are different sizes (my hubs would be swallowed up in an adult large!), so look for a pattern that comes in a variety of sizes.