You want to crochet something as a gift, for selling or even for yourself? A hat seems an excellent idea and doesn't take too long to make. If you are following a pattern, the calculations will have all been done for you. But, what if you want to use different stitches and make up your own design? This chart is a good starting point but there are still a few things to consider when reading the figures here. Different yarn can produce different results, even if you are using the same hook size. If you know who you are making the hat for, it may be worth measuring the circumference of their head (around the largest part) and asking how long they want it. Just remember, these figures are a guide only and some people prefer a tighter or looser fitting hat.