Men's jacket sizes

What does a 40 in a men's jacket mean?

In the US and UK convention a men's jacket marked 40 is drafted for a 40 inch body chest. The number describes the person, not the garment. A 38 is cut for a 38 inch chest and a 42 for a 42 inch chest, and the ladder normally moves in two inch steps.

The letter after the number is a length class, not a width. R is regular, S is short, L is long. A 40R and a 40L are drafted around the same chest and differ in body length and sleeve length.

Two consequences follow. Because the number names a body, the garment itself is always cut wider than the number so the wearer can move and layer. And because no standard obliges a brand to honour the convention, a 40 from one label and a 40 from another can differ across the chest by an inch or more.

This table is the naming convention and nothing more. It is not a measurement of any particular jacket, and ShirtMath does not publish a jacket table of its own, because the garment figures that would fill one have to come from each brand's own specification sheet.

How do you measure a jacket flat?

A tape on the garment settles what a label cannot. The method is the same one used for shirts, and the longer version of it is in the guide to measuring a shirt; the pit to pit guide covers the single line that matters most.

1. Close it and lay it flat — button or zip the jacket fully, face up, on a hard flat surface, and smooth the chest without pulling it.
2. Measure armhole to armhole — run the tape straight across the chest at the point where the sleeve seam meets the body, and keep it level rather than following the curve of the panel.
3. Read it twice — record the figure, lift the tape, and take it again. Two readings that disagree by more than a quarter inch mean the garment shifted, not that the jacket is between sizes.
4. Double the figure — a flat reading is half the circumference. Double it before you compare it with any body chest number on a size label.
5. Measure a jacket that already fits you — a jacket you wear happily is the only reference that accounts for your shoulders, your posture and the layers you actually put under it.

Comparing a new jacket against one you own is more reliable than comparing either against a label, because both sides of that comparison are garment measurements taken the same way.

How does a jacket chest relate to a shirt chest?

The two numbers are not the same kind of number. Shirts are normally published flat, as half the chest measured pit to pit. Jackets are normally sold by the body chest they are drafted for. To put them side by side, double the shirt figure first.

A unisex Medium tee at 20.0" flat comes to 40.0" round, which reads like a 40 jacket but means something different: the tee figure is the garment, the jacket figure is the wearer. A jacket sized 40 has to be larger than 40" round, or it would not close over a 40 inch chest at all.

The ladies' cut in that table shows how far a letter can drift. The Gildan G500L publishes a 17.5" chest at Medium against 20.0" for the unisex G500 and 20.5" for the Next Level 3600, a gap of up to 3 inches on the same letter, and the same 3 inch gap appears at Large. Jacket sizing avoids part of this problem by naming a number of inches instead of a letter, but only part of it, because the number still describes a body and the cut still varies.

How much layering allowance should a jacket have?

There is no single correct allowance, and any page that gives you one figure for it is guessing on your behalf. The allowance a jacket needs is set by what goes underneath, and that varies from a single tee to a thick knit.

The measurable version of the question is simple. Take the thickest garment you expect to wear under the jacket, measure its flat chest, double it, and treat that circumference as the floor the jacket has to clear with movement room left over. A sweatshirt that measures 22.0" flat on your own tape is already 44.0" round before the jacket goes over it, which is why a nominal 40 jacket bought for a 40 inch chest can feel impossible over winter layers and fine over a shirt.

Why does the same jacket number fit differently between brands?

Because the number is an input to a pattern, not an output of one. Two brands can both draft a 40 for a 40 inch chest and still choose different amounts of room, different armhole heights and different shoulder widths, and all three change how the same nominal size wears.

Letter sizing on tops has drifted for the same reason, which is the subject of the vanity sizing guide. Numbered jacket sizing is more honest than a bare S, M or L because it at least names a unit, but a named unit is still not a measured garment. If you are moving between labels, the question in what size am I in a different brand is answered by measurements on both sides, never by matching the number.

What ShirtMath can and cannot do here

ShirtMath compares garment measurements, and the live scanner reads a shirt laid flat with a scale card in the frame. Roughly half of its readings land within an inch of a tape, sleeve opening and bicep are the least reliable readings, and anything it cannot read confidently is left blank rather than filled in.

Jackets are not part of that today. This page is a reference: the tape and the brand's own specification sheet are the tools for jacket sizing, and the arithmetic above is what connects them. The methodology page sets out how published figures are stored and compared.

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