You can use hat size charts from so many different websites including ours here, but the best answer is to just go try on the hat itself. I say this because hat companies usually makes their hats from a couple of different factories and each factory might make the hats a little different depending on their production tolerance levels. And even if they made all their hats from the same factory, there will ALWAYS be some production differences from one hat to the next in terms of fit even if you can visually or feel the difference. I would equate this to buying pants. Even if you're a certain size, it's always best to try on every pant you want to buy.
Obviously, the issue of hats fitting correct pertains to mainly Fitted hats. This is due to the process of sewing the taping and sweatband inside the hat. The taping and sweatband sizing is what makes or breaks a hat's sizing. I would suggest for you to never judge if a hat fits just off the weight of looking at the size label, TRY ON THE HAT!
Since we know this problem is far too common, we've combated this by providing a generous Return/Exchange Policy for not just online orders but in-stores.