Determining your ultimate underwear size involves a few simple steps. Start by measuring your waist using a flexible tape measure; wrap it around your waist where you typically wear your underwear. For the best fit, refer to a size chart that correlates your measurement with various brands, as sizing can vary. Common mistakes include not measuring snugly or assuming all brands fit the same. Always try on different styles, like boxers or briefs, to find what feels most comfortable for you. With these tips, you’ll confidently select the right size!