You can find online belt sizing charts on eBay.com and a clothing sizing resource called Sizecharter.com. You can also find charts specific to brand on the sites of individual belt makers, such as Luxurycustomaccessories.com.
It is important to remember that belt sizing is different from pants sizing. Belt size, which is usually measured in inches, is generally 2 inches larger than waist and hip measurements. Also, designers size belts to the middle hole, meaning there may be some overlap of what sizes you can wear when considering you can tighten or loosen to another notch. Unlike clothing, belts tend to be more universal in size because designers measure them in precise inches.