Quick Answer: Shops are 20 to 22C, windless, and you try jackets over one thin layer while standing still. On the street you have a knit underneath, a bag on your shoulder, wind, and you are walking. The four tests that predict street performance: cross your arms, put a bag on, do the collar up, and walk ten steps. All four take under a minute and catch nearly every jacket that fails later.

It fitted beautifully. You turned in the mirror, the shoulders sat perfectly, the length was exactly right.

Then you wore it on a Tuesday over an actual jumper, with an actual bag, in actual wind — and it was tight across the back, it rode up at the shoulder, and the collar would not stay down.

Nothing about the jacket changed. Everything about the conditions did.

What is different about the shop?

Four variables, and shops control all of them in the jacket’s favour.

Temperature. Retail floors run at 20 to 22C, which means you are trying winter outerwear over a t-shirt. In real use there is a knit under it, and a mid-weight jumper adds 3 to 5cm to your chest circumference. That is the difference between comfortable and pulling.

Stillness. You stand and turn. You do not reach, swing your arms, or carry anything. A jacket that fits statically can bind through the armhole the moment you move properly.

No wind. Wind reveals whether a collar stands, whether a hem flaps, and whether the front closure actually closes. A jacket that looks great open in a windless room is a different object on an exposed street.

No bag. A shoulder bag or a cross-body compresses one shoulder and drags the jacket sideways. This is the most-ignored variable and the one that ruins the most jackets, particularly anything with a soft or unstructured shoulder.

What are the four tests that actually predict it?

Each one maps to a variable the shop has removed.

Cross your arms fully, hands to opposite shoulders. This tests the back width and the armhole. If it pulls across the upper back or you feel the armhole grip, the jacket is too narrow through the back — which cannot be altered on most jackets, because the back is cut in one or two panels with minimal allowance.

Put your actual bag on, or ask for one. Look at the shoulder line from the side. A jacket with a soft shoulder will collapse under a strap; a structured one holds. Also check whether the strap sits on the sleeve head seam, which is where wear shows first.

Do the collar and top closure up. Half of the jackets that look good open do not close comfortably at the neck, and a coat you can never fasten is a three-season coat sold as a four-season one.

Walk ten steps, including turning around. Watch the hem and the sleeve. A hem that swings into the back of your legs is too full for the length; a sleeve that rides above the wrist bone when your arm swings forward is too short — sleeves should be measured with the arm slightly bent, not hanging straight.

How much room should there actually be?

Enough for the layers you will wear, measured rather than guessed.

Chest: room for a fist between the closed jacket and your chest, over a mid-weight knit. Less than that and it will pull when you move; much more and it reads oversized rather than roomy.

Shoulder seam at the edge of your shoulder, or slightly beyond. For a tailored jacket, exactly at the bone. For a coat worn over knitwear, up to 1.5cm beyond is correct and intentional — a shoulder seam sitting inboard of your actual shoulder is the clearest sign a coat is too small, and it is the one alteration no tailor can perform.

Sleeve to the wrist bone with a bent arm. Long enough to cover the wrist when reaching forward. Sleeve length is one of the few things that alters easily and cheaply, so do not reject a good jacket over it.

Armhole high, not low. A high armhole allows more arm movement, counterintuitively. A low, dropped armhole means lifting your arm lifts the entire jacket body, which is why cheap coats feel like they are climbing off you.

Length: it should end at a natural point on the body — mid-thigh, just above the knee, or below the knee. Hems that end at the widest part of the hip or exactly at the knee are the two lengths that most often read as wrong without an obvious reason.

5 Mistakes Everyone Makes Buying Outerwear

1. Trying it on over a t-shirt in September. Take a jumper with you, or wear one. This one habit prevents more bad outerwear purchases than everything else combined.

2. Judging it only with the front open. Most coats are cut and photographed to look best open. Fasten it fully and look again — that is how it will be worn on the coldest days, which are the days you bought it for.

3. Ignoring the lining fabric. A lining that is not slippery makes the coat catch on your sleeves every time you put it on, and you will find it annoying every single day. Run your hand inside the sleeve; it should slide.

4. Buying a shoulder you cannot alter. Sleeves, hem and even the waist can be adjusted. The shoulder and the back width essentially cannot, so those two need to be right on the rail. The third mistake costs comfort; this one costs the whole jacket.

5. Choosing a colour that only works with one outfit. Outerwear is worn over everything, so it needs to work with your most-worn trousers and your most-worn shoes. Check it against what you actually wear, not against the one outfit you imagined while buying it.

FAQ: Outerwear Fit Questions, Answered

Should I size up so it fits over a chunky knit?

Usually no. Sizing up widens the shoulder, which is the one dimension that should stay accurate. Instead look for a coat cut with a fuller body and a high armhole in your normal size — the room you need is in the circumference and the armhole, not the shoulder width.

What can a tailor realistically fix?

Sleeve length, hem length, taking in the waist and the side seams, and moving buttons. What they cannot fix affordably: shoulder width, back width, armhole depth, and anything with a bonded or taped seam. Buy for the second list and alter the first.

How do I check a coat will survive a real winter?

Look for a lining, a decent weight of face fabric, and check whether the closure overlaps generously at the front. A single-breasted coat with a narrow overlap will let wind straight through the button gap regardless of how warm the fabric is.

TL;DR:

• The shop removes four variables: temperature, movement, wind, and your bag. All four change the fit.
• Four tests, under a minute: cross your arms, add your bag, fasten the collar, walk ten steps.
• Room means a fist at the chest over a knit, a high armhole, and a shoulder seam at or just beyond the bone.
• Alterable: sleeves, hem, waist. Not alterable: shoulder and back width — get those right on the rail.

Outerwear is the highest-use category in a wardrobe and the one most often bought under the least realistic conditions. Spending sixty extra seconds in the fitting room doing four unglamorous movements is a poor trade for nothing and an excellent trade for a coat you will wear four hundred times. Take the jumper with you next time; it changes what you buy more than any amount of reading about fit does.