US Size 40 typically refers to a men's clothing size, specifically for jackets and coats. This size indicates a chest measurement of approximately 40 inches. In the context of men's suits, a size 40 is often categorized as a "Medium" or "Regular" fit, suitable for individuals with an average build. In women's clothing, US Size 40 is less common and may correspond to a larger size, often around a size 14 or 16, depending on the brand. It's important to note that sizing can vary between different manufacturers, so trying on clothing is recommended for the best fit.