How to work out your actual size when every brand disagrees

There is no legal or industry definition of a clothing size. A medium is whatever a brand decides a medium is, and because brands design for the body they imagine their customer has, the same label can differ by several centimetres between two shops on the same street. This is not a conspiracy or a failure — it is simply the absence of a standard, and it has been the situation for as long as ready-to-wear has existed.

The practical consequence is that shopping by size label is guesswork, and shopping online by size label is expensive guesswork. The alternative takes twenty minutes once and then works indefinitely: measure yourself, write it down, and learn to read the chart that most retailers already publish.

Measure yourself once, properly

You need a soft tape measure and, ideally, another person, because measuring your own back accurately is close to impossible. Wear thin clothing or none, stand normally rather than at attention, and let the tape sit snug without compressing.

• Chest or bust: around the fullest part, tape level all the way round and under the arms.
• Waist: at the natural crease when you bend sideways, not where your trousers sit.
• Hips: around the fullest part, usually a little below the hip bone.
• Shoulder: from the bony point of one shoulder to the other, across the back.
• Inside leg: from crotch to the floor, barefoot.
• Sleeve: from the shoulder point, over a slightly bent elbow, to the wrist bone.

Write these somewhere you will find them again. Re-measure once a year, or after any significant change; bodies move, and a chart from four years ago produces the same disappointment as no chart at all.

Body measurements versus garment measurements

This is the distinction that unlocks the whole thing, and retailers are inconsistent about which they publish.

A body measurement chart says what size to buy if your chest measures a certain amount. It has ease built in — the extra room the designer intended — but you cannot see how much. A garment measurement chart tells you how wide the actual shirt is, flat, across the chest. That is far more useful, because you can compare it to something you already own that fits.

The single best trick in online clothes shopping is to measure a garment you already own and love, laid flat, and compare it to the retailer’s garment chart. It removes the guesswork about ease entirely, because you are comparing like with like rather than trying to imagine how a designer intended something to sit.

Measuring a flat garment is worth doing carefully, because the conventions are not obvious. Chest is measured across the garment from armpit seam to armpit seam and then doubled. Length runs from the highest point of the shoulder straight down the back, not along the curve. Sleeve is taken from the shoulder seam to the cuff edge. Retailers use these same conventions, so as long as you match them the comparison is reliable.

Keep a short list of the garments you have measured. Over time it becomes a reference of its own — this shirt fits, it is 54cm across the chest, therefore anything between 53 and 56 will probably work. That single number does more for online shopping than any brand-specific size guide, because it is anchored to something real rather than to somebody’s idea of a medium.

Which measurement decides the size

The shoulders rule is the most valuable line in that table. A shirt or jacket can be taken in at the waist, shortened at the sleeve and hemmed at the body for modest money. Moving a shoulder seam means dismantling the garment, which costs more than most garments are worth. Buy for the shoulder and alter the rest.

Reading between the sizes

Almost nobody matches a size chart exactly, and the usual advice — size up — is only right about half the time. Which way to go depends on what the excess is doing.

If you are between sizes at the chest but not the shoulders, take the smaller and let it out or accept the closer fit; the alternative is a garment that hangs off the shoulder line and looks wrong regardless of how comfortable it is. If you are between sizes at the waist, take the larger and have it taken in, which is a straightforward alteration.

Fabric matters here too. Something with elastane will accommodate a centimetre or two; a rigid woven cotton will not. Read the composition before deciding which way to round.

Shrinkage is the variable people forget when rounding down. Untreated cotton and linen can lose a noticeable amount on a first hot wash, and knitwear can change shape rather than size. Where a garment is not described as pre-shrunk, treating the smaller size as marginal rather than merely snug is the safer reading — and washing cool is the cheapest insurance available.

Making returns work for you

Since sizing is unreliable, the returns policy is doing part of the job a fitting room would. Ordering two adjacent sizes and returning one is a legitimate strategy where returns are free, and it is considerably cheaper than keeping something that does not fit.

Two cautions. Check whether returns are free before ordering multiples, because paid returns turn the strategy into a loss. And check the return window against how long the item takes to arrive — a fortnight from delivery is fine; a fortnight from order can leave you very little time.

What to know before you buy

• Measure once and record it. Twenty minutes that saves years of guessing.
• Prefer garment charts to body charts. They tell you the actual dimensions.
• Measure something you own that fits and compare directly.
• Buy tops for the shoulders and alter the rest.
• Check the fabric composition before deciding which way to round.
• Read the returns terms first if you plan to order two sizes.

Why do sizes vary so much between brands?

Because there is no standard to vary from. Each brand fits its garments on a house model whose proportions reflect its intended customer, and those proportions differ. A brand aimed at a broader build and one aimed at a slimmer one will label very different garments the same way, entirely legitimately.

Does vanity sizing exist?

Sizes have drifted over decades, so a size label from thirty years ago corresponds to a smaller garment than the same label today. Whether that is deliberate flattery or simply brands tracking changing populations is arguable. Either way it is another reason to work from measurements rather than labels.

Is it worth having clothes altered?

Frequently, and it is under-used. Shortening sleeves or trousers and taking in a waist are inexpensive and transform how a garment sits. The rule is to buy for the dimension that cannot be altered and adjust the rest — which is the same principle behind the white shirt test and the boot resoling question.

None of this makes clothes shopping infallible, but it moves it from chance to probability. A recorded set of measurements, a habit of measuring a garment you already own, and a rule about which dimension decides the size will get you the right thing far more often than reading the label ever did.