These sizes are made a little smaller than the usual American sizes. So, if you usually wear an XS in America, you may want to order a size S, unless you like a tight fit. If you are really petite and the usual XS is loose on you, you would definitely stick to an XS.
We would also recommend going a size up if you have a bra size of a full B or C cup. For example, if you usually wear a medium but you are a 36C, you would want to order a large size. For those who like their tank tops longer, it is also a good idea to order a size up from your usual size. The tank tops are made of 100% cotton that stretches but you don't want it too stretched out for the sake of the design on the front of the top.
Tip: If you want the tank top to shrink a little for a tighter fit, you can wash it in hot or warm water separately and use tumble dry. If you don’t want it to shrink, wash it in cold water and tumble dry low.