Overcoats, sized to go over a suit jacket

Will my suit jacket fit underneath

This is the question, and nobody answers it properly.

Every guide online treats it as styling: what colour, what length, does it look right. That is not what is being asked. The person asking is standing in front of a coat wondering whether to take their usual 40 or move to a 42.

The answer is a measurement, not an adjective. A coat that fits comfortably over a shirt will not fit over a structured shoulder, because a suit jacket adds bulk at exactly the point where a coat is least forgiving.

The practical rule: if the coat is going over tailoring, you need roughly two inches more chest than your suit jacket size. If it is going over a jumper, one inch is usually enough. If it is only ever going over a shirt, your normal size is right.

How long should it be

There is no standard, and the honest position is that length is a proportion question rather than a rule.

The conventional reference points are just above the knee for a standard overcoat, mid-thigh for a car coat, and below the knee for a full-length. What actually decides it is your height and what you wear underneath: a coat has to clear a suit jacket's hem by enough that the jacket is not peeking out, which for most people puts the minimum at a couple of inches below the seat.

Shorter coats read as more casual and are easier to drive and cycle in. Longer coats are warmer and look better over tailoring.

Overcoat, topcoat, car coat

We looked for an authoritative source separating these three and did not find one worth citing. The conventional distinction is cloth weight and length, with a topcoat being lighter and shorter than an overcoat, but we are not publishing figures we cannot stand behind.

What we can say is that the terms are used loosely in the trade and that judging the garment beats judging the label.

Down, and what fill power actually tells you

If any of these are down-filled, the number on the tag is fill power, measured in cubic inches of loft produced by one ounce of down. An ounce of down is put in a cylinder, a weighted disc is placed on top, and the compression is read after a minute. Four hundred and fifty cubic inches per ounce is 450 fill power.

The bands: 300 to 500 is poor, 600 to 650 good, 700 to 750 very good, 800 to 950 and above excellent. Feathers sit around 300. Premium goose down reaches about 900, which is roughly as good as down gets.

Here is the correction almost no retailer makes: fill power does not tell you how warm the coat is. It tells you the quality of the down, not how much of it is in there. A heavy load of 600 fill will out-warm a light load of 800 fill. Warmth comes from fill power multiplied by fill weight, and most brands only publish the first number.

Rain

Hydrostatic head is the measure of waterproofing, tested under ISO 811 and ISO 1420. The European rain standard, BS EN 343, sets its requirement by class, starting at around 82cm of water gauge. Outdoor retailers quote a separate and more generous millimetre scale, where 5,000mm is basic and 20,000mm is high performance, and the two are not comparable.

A wool overcoat has no hydrostatic head rating at all. It is not a rain garment, it will wet out in sustained rain, and it takes a long time to dry. In Belfast that is worth knowing before you spend.