Layering under a coat without losing the line

Every layer you put under a coat adds twice its thickness to the circumference you need, because the coat has to go round both sides of you. A 5mm jumper is 3cm of chest measurement gone. That arithmetic is why a coat that looked sharp over a shirt in September looks strained over knitwear in January, and why the usual advice to size up is only half right: a coat sized up has a wider chest and also a wider shoulder, which is the one dimension you cannot afford to get wrong. What follows is where the room has to come from instead.

The arithmetic, which is unforgiving

Circumference grows by pi times the added diameter, which for practical purposes means a layer of a given thickness adds about six times that thickness to the measurement around you. A thin merino jumper at 2mm adds roughly 1.2cm. A mid weight lambswool at 5mm adds about 3cm. A chunky cable knit or a sweatshirt fleece at 10mm adds 6cm, which is most of a size.

That is why a coat fitted over a shirt will not close comfortably over a jumper, and why the strain shows as diagonal lines from the button across to the opposite hip rather than as anything you can feel.

The useful consequence: measure a coat flat across the chest, double it, and compare it to your own chest measurement plus the allowance your intended layers need. A coat worn over knitwear wants something like 12cm to 20cm of total ease over your bare chest measurement. Over a tailored jacket, more.

Where the bulk actually shows

Not at the chest. The chest is the number everybody checks and the place where extra fabric is least visible, because the coat front hangs away from the body anyway.

It shows at the armhole first. A jumper adds thickness at the exact point where your arm meets your torso, which is the tightest part of any coat, and when the armhole is full the sleeve stops hanging vertically and starts standing out from the body. That is the single most visible sign of a coat with too much under it.

Then the shoulder. A thick knit sits on top of your shoulder and effectively raises it, so a coat shoulder that was correct now sits slightly high and the collar lifts away from your neck at the back. A gap behind the collar is almost always this, or a shoulder that was wrong to begin with.

Then the upper sleeve, which has to contain a jumper sleeve plus your arm, and which on a slim coat is the first thing to feel restrictive when you reach forward.

Buy warmth that is not thickness

Insulation is trapped air, and some fibres trap more air per millimetre than others. Merino wool in a fine gauge is the best value in the category: a thin merino crew is warmer than a thick cotton sweatshirt and takes up a fraction of the room. Cashmere is warmer again per millimetre and wears out faster.

Cotton is the worst choice for a mid layer under a coat. It is bulky for its warmth, it holds sweat, and once damp it actively cools you. A cotton hoodie under a tailored coat is the combination that both looks wrong and performs badly.

The thing that changes the most for the least thickness is a base layer: a fine merino long sleeve against the skin, under a shirt, adds a millimetre and a half and a surprising amount of warmth. If a coat fits you over one jumper and not two, this is the layer to add.

And a scarf does more than any of it. The neck and upper chest lose heat fast, a scarf is outside the coat so it costs nothing in fit, and it closes the gap at the collar that lets warm air out.

Coat shapes that take layers, and shapes that do not

A deliberately oversized cut is designed around the problem. The armhole is cut deep and low, the shoulder seam sits down the arm so a raised shoulder underneath does not disturb it, and there is enough width in the body that a chunky knit is invisible. The Oversized Long Coat is double breasted and cut large on purpose, which is the easiest kind of coat to layer under.

A double breasted front helps on its own, independent of the cut, because the overlap is wide and the button position is set for a body with something under it. The front also closes in two places, which distributes the pull.

A tailored single breasted shape with a defined shoulder is the hardest. The Suit Trench is built to look like outerwear over a jacket, and it is sized with that in mind, but a sharply built shoulder shows every error, including one introduced by a jumper.

A belted coat is a special case. The Belted Trench has no fixed waist measurement, so you can let the belt out over a thick layer, but the shoulder and armhole still have to be right and no amount of belt adjustment helps there.

Keeping the line when the coat is already bought

Change the mid layer rather than the coat. Swap the chunky knit for a fine gauge one and add a base layer, and you can usually recover 3cm or 4cm of circumference with no loss of warmth.

Move the bulk below the armhole. A high necked fine knit plus a scarf keeps the heat where you lose it and leaves the shoulder alone. A gilet, worn under the coat, is the one thick layer that does not affect the sleeve or the armhole, because it has no sleeves to fill them.

Leave the coat open. A long coat worn open hangs from the shoulders and reads as intended; a long coat buttoned over too much fabric reads as too small. This is a real option, not a concession, and it is how most belted and double breasted coats are actually worn.

And check the sleeve length again. Knitwear underneath pushes the coat sleeve up your wrist by a centimetre or two, which is enough to make a correctly hemmed sleeve look short. If you only ever wear the coat over a jumper, hem the sleeve to that, not to a shirt.

Lining, and why some coats feel warmer than their fabric

A smooth lining does nothing for insulation and a great deal for how a coat goes on over knitwear. A bare or unlined coat drags across a jumper, pulls the sleeve of it up your arm, and feels tight when it is not. Half the complaints about a coat being restrictive are friction, not measurement.

A high shine or coated outer such as The Patent Trench blocks wind completely, which makes it feel warmer than a woollen coat of the same weight in gusty weather and colder in still cold, because it holds no air itself. The insulation has to come from underneath, which means the fit allowance matters more on that kind of coat, not less.

If you are choosing between two coats and one has a full lining with a vent and the other does not, the lined one will take a layer underneath better even where the measurements are the same.

Questions people actually ask

How much bigger should a coat be than your chest?

Over a shirt, about 8cm to 12cm more than your bare chest measurement. Over knitwear, 12cm to 20cm. Over a tailored jacket, more again, plus two to four centimetres across the shoulder.

Should I size up a coat for layering?

Only if the shoulder still fits in the larger size. Sizing up widens the shoulder as well as the chest, and a shoulder seam that has slipped down your arm cannot be altered affordably. Change the mid layer first.

What is the warmest thin layer to wear under a coat?

A fine gauge merino base layer against the skin. It adds a millimetre or two of thickness and a lot of warmth, and unlike cotton it keeps working when damp. A scarf is the next best, because it sits outside the coat and costs nothing in fit.

Why does my coat collar gape at the back over a jumper?

A thick knit sits on top of your shoulders and effectively raises them, so the coat rides up slightly and the collar lifts off your neck. A thinner mid layer usually fixes it. If it gapes over a shirt too, the shoulders are wrong.