Winter coats are cut specifically with the idea that you'll be wearing other items of clothing underneath them", says fashion editor, Rivkie Baum. "Meaning they should already be a little roomier than a t-shirt in the same size. To that end, you do not need to size up when buying a winter coat. If buying a tailored design, you may find it a little too fitted to wear bulky sweaters underneath. If you are tempted to size up, make sure to look at the shoulders of the coat as a size guide when trying on. The shoulder seams should still sit on your shoulders, when it comes to getting a smart but well fitting coat.