Do you dislike ordering clothes off the Internet, because you have to guess what size might fit, and order two sizes just to make sure, all the time knowing you’ll have to ship at least one of them back? Well you’ll love this: There’s a new computer program that can calculate your correct size,  even for a brand you’ve never bought before, and it’s based on what you already own in your closet!

Retail experts say that 30% of online sales have to be returned, and the number one reason is because they don’t fit. Sizes are especially difficult in women’ clothing, where a size 4 in one brand can be a size 8 in another. Even when shoppers input their own measurements, they often measure incorrectly, or flat-out pretend they’re smaller.

But you can’t trick the new True Fit technology.

Here’s how it works: You create a profile that includes your age, height, and weight, and answer a few questions about your body shape, like whether your belly pooches out high, or low. You also identify brands, styles and sizes from your own closet that fit well. The program then calculates your size and shape based on that information, and can predict your size in 350 different fashion brand, with more labels being added all the time.

You can set up your True Fit profile at Macys.com. You’ll be able to get sizing recommendations for the brands they carry, and use that same profile to get sizing recommendations at other retail websites, too. The program will even track what you buy online, and what you return, so the more you shop using True Fit, the better the size predictions will become.