Just click the 'Check my BELL ST size' button on any product page and follow the instructions. The sizing tool will ask you to input your body measurements.
For accurate results, take your body measurements on bare skin using a soft measuring tape. Be sure to keep the tape nice & level and not too tight or too loose.
The sizing tool will then recommend your best size. And you only need to fill it out once - the tool will automatically adjust its recommended size according to the jean you're viewing.
