A 30-inch waist typically equates to a size 10 in United States measurements. Every manufacturer is different, so it is recommended to review the size charts that refer to specific retailers before purchasing clothing.
For example, the size charts provided by J Crew and Levi's both list a 30-inch waist as a size 10, whereas Gap considers a size 10 to be a 29.5-inch waist and a size 12 to be a 30.75-inch waist. These small discrepancies can make the difference when deciding on the right fit. It is important to access the sizing chart online or refer to one in stores.