Why Do All Clothes Brands Differ in Sizing?

I cannot tell you how many times I've stood in a changing room wearing three different sizes of the same type of trouser, from three different shops, all technically labelled a "12," thinking: who on earth is this system designed for? The short answer: it was never really designed for you. It was designed for the brand.

A Brief History of the Sizing Chaos

Standardised clothing sizes were introduced in the early 20th century, largely driven by the military and ready-to-wear manufacturing. But the measurements used were based on very limited datasets — often taken from a narrow demographic — and were never updated consistently as body shapes and populations changed. By the time high street fashion exploded in the 1960s and 70s, each manufacturer had essentially gone off and done their own thing.

In the UK, the British Standards Institution made attempts to standardise women's sizing, but compliance was always voluntary. Brands simply weren't required to follow it. So they didn't — or they adapted it to suit their own business needs.

The "Vanity Sizing" Effect

Here's where it gets a bit uncomfortable: many brands deliberately size up their garments while keeping the label number the same (or even smaller). It's called vanity sizing, and it's been rampant for decades. The idea is that if you pick up a pair of jeans and find you're a size 10 rather than your usual 12, you feel great — and you're more likely to buy.

"A size 12 today is not the same as a size 12 twenty years ago. The label hasn't changed. The measurements have."

The brands doing this tend to be the ones targeting aspirational consumers. On the flip side, some labels — particularly those manufacturing in parts of Asia for a global market — run notably small. I once ordered a blazer from a brand I'd never tried before, sized up two sizes based on reviews, and it was still snug. Embarrassing? A little. Useful lesson? Absolutely.

Manufacturing Country Makes a Massive Difference

Where a garment is made has a huge impact on how it fits. Factories in different countries use different base patterns — called "blocks" — as their starting point. A brand manufacturing in Italy will work from a very different block to one manufacturing in Bangladesh or China. Those foundational patterns reflect the average body proportions of the workforce producing them, or of the brand's historic customer base, and they don't always translate across body types.

Zara is notorious for sizing small — particularly in tops and dresses. They manufacture largely in Spain and Portugal with a European fit in mind. Going up one size is almost always my advice.
M&S is generally considered one of the more consistent and generous sizing brands on the UK high street — reliable for a reason, and their fit has improved enormously in recent years.
ASOS carries hundreds of own-brand and third-party labels, so sizing varies wildly within the same website. Their own-brand tends to be consistent, but always check the size guide per item.
Mango runs on the smaller side, similar to Zara — expect European proportions. I'd always recommend trying before buying or checking returns policies.
Next is generally true-to-size and one of the better high street options for a reliable fit across styles, which is why they've built such loyal customers over the years.
H&M can be inconsistent depending on the range — their basics tend to run small, while their premium lines sit a touch more generously. It's genuinely pot luck sometimes.
River Island fits fairly true to size but cuts slim — particularly in jeans and trousers — so if you're between sizes, always go up.

Premium and Designer: Does Paying More Mean Better Sizing?

Not necessarily better, but often more consistent — because higher-end brands tend to use their own bespoke fit models and hold their patterns to a tighter standard. That said, they come with their own quirks.

Reiss cuts quite slim and tailored — wonderful quality, but if you have any curves, be prepared to size up. Their dresses in particular can be unforgiving around the hips.
Me&Em has fantastic sizing consistency and caters brilliantly to women who aren't a sample size — one of the premium brands I genuinely trust for an accurate fit.
Whistles sizes are generally reliable and sit true-to-fit — a good benchmark if you're new to the brand.

At the luxury end, Max Mara runs distinctly large in their classic coats and knitwear (intentionally — their pieces are designed to layer), while other houses like Sandro size very small indeed. There's no universal luxury standard either.

Two Brands Worth Knowing

If you haven't come across Sézane yet — a French label now widely available in the UK — be aware they size French-small. Go up at least one, sometimes two sizes. The quality is brilliant, the style is effortless, but the sizing will catch you out if you're not prepared. Equally, Kitri, an independent London-based brand, is a joy — their sizing is transparent, honest, and they include detailed fit notes on every product page. A rare and refreshing thing.

So What Can You Actually Do About It?

Always take your own measurements (bust, waist, hips) and keep them to hand when shopping online
Ignore the number on the label — it genuinely means nothing across brands
Read customer reviews specifically for fit comments before buying
Check a brand's individual size guide every time — don't assume last season's 12 fits this season's 12 from the same brand
When in doubt, order two sizes and return the one that doesn't fit

Stop Guessing Your Size — Let Tellar Do It For You

This is exactly the problem Tellar was built to solve. We're the UK's leading sizing tool — and we're completely free.

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