Bring the most frequent combination

Choose the layers you expect to wear most often rather than automatically selecting the thickest jumper in your wardrobe. If you regularly wear a blazer underneath, include it. Settle the lower sleeves before assessing the coat. A bunched blouse sleeve can feel like insufficient room when it is simply twisted out of place.

Fasten the coat normally and reach forwards. Notice whether the shoulders restrict movement or the buttons sit under visible tension. Open it and compare. A design comfortable only when open may not suit days when you need the coat closed.

Assess more than room around the body

A coat can be generous through the body but still have close-fitting sleeves. Check the elbow with the intended knitwear underneath. Bend both arms and hold a bag as you would while travelling. If the layers bunch heavily, another sleeve shape may help more than extra room everywhere.

See whether the collar remains comfortable with a scarf or roll-neck. A hood compressed beneath the coat collar changes its position. Arrange it as you will actually use it rather than temporarily hiding it for the mirror. The coat needs to work with the garment you own.

Try a lighter layer as well

If the coat is intended for several seasons, repeat the fitting over a thinner combination. Some additional room may be welcome then. Still check whether it slides substantially backwards or whether the sleeves and shoulders shift in a way you dislike. A wide range of use involves a compromise between different layers.

Walk a little and sit with the coat fastened if that reflects your routine. Alternatively, open it for sitting as you normally would. Note which arrangement worked. Later, you will remember whether the coat was chosen mainly for thick knitwear, a work blazer or lighter transitional outfits.

Compare a larger size deliberately

A larger size often changes length, shoulders and pocket position together. Do not assess only the extra room around the upper body. If the next size creates other problems, look for a cut with more space where you actually need it. The label is a selection aid, not the purpose of the fitting.

Keep your shoes and usual bag position in the final comparison. A coat is the outer layer over a complete outfit; its suitability cannot be established entirely through an isolated fitting over a thin T-shirt.