A: Hello. Generally, a person's numerical vest size is the measurement at the widest point on their chest. For example, if you were to measure the widest point (presumably across your breasts) and that measurement were 38, you would order a size 38 vest. The exception would be if you are aware that your midsection is substantially bigger around than your chest, you would want to upsize by a size or two to compensate. Hope that helps!