If you like to crochet hats, this chart will help you with your finished measurements. I always come back to this chart to make sure my hat is the right size for the person I am making it for. Remember, these sizes are approximate. If you are making a crochet hat, it will stretch up to 2" depending on what kind of yarn you are using. I find that acrylic and wool stretch more than cotton.
I also have a chart for shoe sizes, and a crochet hook conversion chart.