Manufacturers have sizing guides, however, how accurate these guides are is another issue entirely. Once you’ve decided on what jacket you want, check reviews to see if other buyers found the jacket to be true to the size guide, bigger than advertised, or smaller than advertised. Then you should measure yourself, and make your selection based on that. Or, if possible, go to a store and try them on before buying. It’s better to have a jacket that’s slightly too big than too small as, if you need to add extra layers, the jacket might become too restrictive. But keep in mind that if the jacket is much too big, its protective properties will be diminished.