Determine your standard coat size. If you wear a overcoat size 36, you'll want an overcoat size 36. Overcoats are designed to fit over heavy, thick layers of clothing and the sizes are supposed to be fitted accordingly. If you try on a overcoat that has the same tag size as your regular overcoat size, it should fit. If the overcoat doesn't try a half-size to a size larger
Check the overcoat size you are purchasing fits you. Try it on. If the overcoat doesn't fit comfortably or if you don't have enough room to move, obviously you'll want the next size up
Measure yourself if you can't seem to find an overcoat that fits properly and matches your normal coat size. If you are a size 36 in the waist and a size 40 in the shoulders, try wearing a normal 38-size coat. If no overcoat coincides with your normal coat size, you might be wearing the wrong size
Determine what you'll most often be wearing under the overcoat, be it your business suit, your winter coat, a jacket or just your normal street clothes. This will make a difference in the size you need. If you normally wear street clothes under an overcoat, you'll want a size that fits you snugly. If you wear your suit under this coat, it may be a little tight, but it will still be the proper size
Find a chart that will estimate the overcoat size that you need. Most charts will base themselves upon your normal coat size, your height and most times your weight. Find your expected size and see if that overcoat fits. If the overcoat doesn't fit, go one size bigger or smaller depending on why the coat isn't fitting correctly. View an overcoat sizing chart in the Resources section