I own 3 vests in various colors from different companies.  In all of them, I wear a small.  I ordered this in size small and when it arrived, it took me over 5 min to unzip it as the zipper would only move a few inches.  Then, there was absolutely no way that I could fit in it.  It was several inches too slim!  Since my sleeveless shirt size is usually a 4/6, I did not expect this.  It was lightweight and comfortable, but this one appeared to be a "child" size.  It is not even close to US sizes, so beware!