Measure Your Head
Another valid method to determine cap size is to try on a cap and check the size. If you don't own a cap, then try on a random cap next time you are visiting a shop that stocks caps. Sizes, even though they do vary depending on the cap maker, are technically universal. You may refer to the sizing chart if you need to convert a medium to an actual cap size.
Please understand that caps are not unlike other apparel, in that sizes can vary somewhat from one manufacturer to another. We cannot guarantee that by following these instructions any assurance of a perfect fit.