There's no exact conversion method for men to women's underwear sizes, but with a little sense and the right size charts, you can figure out the best size for you. Men's underwear—briefs, boxers and boxer briefs—usually come in standard sizes (small, medium, large, etc.) based on a man's waistline. For example, waist sizes between 26 and 32 inches are usually considered small, 32 to 38 medium and 38 to 44 large. Likewise, women's underwear sizing is typically based on waist size.
Tie a measuring tape around your natural waist, which, for men and women, is where your torso starts to curve in slightly.
Record your waist measurement.
Tie the measuring tape around your hips, which are typically 7 to 8 inches away from your natural waist, and record the measurement. This second measurement will help you decide if you're between sizes.
Consult the size chart of your desired women's underwear brand. Fresh Pair offers a list of men and women's underwear sizes from a variety of brands. Most women's underwear sizes begin around 5, which are meant for women with 26- or 27-inch waists. This would be the equivalent of a men's XS.
Find your waist measurement in inches on the chart. The corresponding size will be your women's underwear size.