Recreate a winter outfit
Bring or wear the jumper you are likely to use on a cold day. Include a scarf if it regularly sits inside the collar. A coat that closes easily over a thin top may become restrictive once the rest of your winter clothing is present.
Fasten the coat fully as designed. Check the chest, upper arms and back without standing unnaturally upright. You need room for your layers, but buying a much larger size is not automatically the answer if the shoulders, sleeves and overall shape become awkward.
Move beyond the mirror
Reach forward, bend your arms and sit if the fitting area allows it. Notice whether the lining pulls, the fastening strains or the collar feels crowded. Test the pockets with your hands in them, since that can change the way the front sits.
Check sleeve length with the intended underlayer. A cuff caught inside the coat sleeve can be annoying even when the coat looks right standing still. If you carry a bag, consider the effect of its strap without assuming a coat should accommodate every possible bulky combination.
Match room to use
A coat worn for short journeys over office clothes may need different space from one worn on long walks over several layers. Choose according to the most likely use, not an imagined worst-case outfit you rarely wear.
Warmth also depends on materials, construction and conditions, so extra size alone is not a reliable warmth strategy. Read the product information and care requirements as well as assessing fit. If two sizes both feel compromised, try a different cut. A coat should let your ordinary winter clothes work together without becoming either a tight shell or an oversized storage space.
