The English speaking world has somehow not managed to agree on terms for many things relating to crocheting and yarn – yarn weights are called different things in the UK, US and Australia/New Zealand and crochet terms are different in the UK and US as well. It’s just the same with how the hook size is given. In the past in the UK Imperial measurements were used, whereas now size is given in Metric measures. I assume it’s the same on the European Continent. Below you can find a guide to converting from one to the other – which is probably mostly useful if you want to convert US sizes into metric and vice versa.