What Is Sizing Like at High Street Stores vs Designer Brands?

High street sizing runs more generously and far less consistently than designer sizing — a high street size 12 is usually cut roomier and more forgiving, while a designer or luxury 12 is graded smaller, sits closer to the body, and is built around one very specific silhouette rather than an "average" shopper. So if you slip into a 10 on the high street and then can't breathe in a designer 12, you haven't changed shape overnight. You've simply met two completely different sizing philosophies.

I've spent years dressing real women in fitting rooms, and the single biggest source of changing-room despair isn't bodies — it's the number sewn into the label. Let me explain what's actually going on, because once you understand it, you stop taking it personally and start shopping much more cleverly.

Why high street sizes feel kinder

High street brands sell to millions, so their patterns are deliberately roomy and forgiving. Three things drive this:

Vanity sizing. Mainstream retailers know we feel better buying a smaller number, so over the years the same "12" has quietly grown. A high street 12 today is often what a 14 was twenty years ago.
Block grading for the "average" body. They grade up and down from one middle pattern, which suits the most common proportions but can leave you swimming in fabric or pulling at the bust if you're outside that average.
Fabric and finish. Plenty of stretch, softer construction and a relaxed cut all add wiggle room — literally.

The upside is comfort and a flattering label. The downside is wild inconsistency between brands, so your "true size" on the high street barely exists.

Why designer sizing runs smaller and stricter

Designer and luxury houses work the opposite way. They're not chasing the masses; they're cutting to an idealised silhouette, usually fitted on a slim sample model, often built on an Italian or French block that's narrower through the shoulder, ribcage and hip.

Closer, more architectural cut. Designer pieces are meant to skim the body, so there's less ease built in. Less ease means less forgiveness.
Fewer sizes, tighter grading. Many luxury lines stop around a UK 16, and the jumps between sizes are smaller and more precise.
European blocks. An Italian 42 or a French 38 doesn't translate neatly to a UK 10 — it usually runs smaller than the conversion chart promises.

It feels stricter because it is. You're paying for precision tailoring, not generous wiggle room.

My own win and my very public fail

The fail first, because it's the more useful lesson. I once ordered a beautiful Italian designer dress online in "my" size 12, fully confident. It arrived, I got the zip halfway up my ribcage, and that was where the dress and I parted ways. Pure Italian grading — I'd have needed to size up twice. Mortifying, and entirely avoidable if I'd checked the actual measurements instead of trusting the number.

The win? A British midi I bought the same month in my genuine measured size that fit perfectly straight out of the bag, no returns, no drama. The difference wasn't my body. It was knowing how each label cuts before I clicked buy.

What this means when you shop

Ignore the number, read the centimetres. Always check the brand's own measurement guide, not the size label.
Size up for European designers and many continental high street labels — assume they run small until proven otherwise.
Buy for your biggest measurement and have the rest tailored. A nipped-in waist is a five-minute alteration; a too-tight bust is a write-off.
Stop brand-hopping in a panic. A 14 in one shop and a 10 in another is normal, not a verdict on you.

Where to shop, by tier

High street — reliable and forgiving

M&S — the gold standard for consistent, true-to-size British fit, with one of the widest size ranges on the high street.
Next — dependable sizing plus petite, tall and curve ranges, so what you order is genuinely what you get.
Boden — generous, forgiving cut that's brilliant if you're between sizes or carry your shape through the hip.
COS — deliberately oversized and roomy, so size down for anything you want fitted.
Uniqlo — globally consistent but cut to a slimmer Japanese block, so I usually size up.
Mango — a sharp European cut that runs small and narrow, particularly across the shoulders.
Seasalt Cornwall — relaxed, comfortable and generous, made for easy everyday wear.
Whistles — refined and slightly slimmer, so it tends to run small for a polished line.

Premium — the sweet spot

Reiss — beautifully consistent, true-to-size tailoring you can trust across the whole range.
Me&Em — designed thoughtfully across multiple heights, with some of the most intentional, predictable sizing around.
Sézane — gorgeous French label that runs small in classic Parisian style, so size up.

Luxury & designer — precision over forgiveness

Max Mara — generous for Italian luxury and often truer to size than its peers, a safe entry point into designer fit.
Totême — clean Scandi minimalism cut slim and small, so it rewards checking measurements first.
By Malene Birger — a slim, elongated Danish cut that suits longer frames and runs narrow.

Two independents worth knowing

Kitri — an independent London label doing small-batch, characterful pieces; sizing is fairly true but worth checking drop by drop.
Lucy & Yak — a Brighton-born independent with a properly size-inclusive range and relaxed, generous fits you can actually live in.

Stop guessing your size — let Tellar do it for you

The whole point of understanding sizing is so you stop wasting time on returns. Tellar.co.uk is the UK's leading sizing tool: match your body to 1,500+ brands instantly and never squint at a size guide again.

Measure once using your bust, waist and hip — or just your existing size in a brand you know.
Use the Store Size Lookup to get your precise size in any brand — COS, Reiss, Everlane, Arket and more.
Always free, no downloads, works straight in your browser.

Plus the Tellar Fashion Hub — a library stacked with free posts from our top stylists. Honest. Unbiased. Independent. Always free. Style advice, top picks and the best brands, all in one place.

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