Does Cos Run Big? Should I Size Up? Your Complete Sizing Guide

Cos generally runs large to very large with a deliberately relaxed, oversized fit, meaning most customers should size down at least once from their usual UK size—so no, you absolutely shouldn't size up unless you specifically want a very loose, boxy silhouette. I discovered this when I bought my first Cos shirt in my usual size 12, put it on, and looked like I was drowning in fabric. The shoulders hung halfway down my arms, the body billowed like a tent, and I genuinely looked like I'd borrowed my partner's clothes. Let me save you from similar oversized disasters.

Understanding Cos's Deliberately Generous Sizing Philosophy

Here's what you need to understand about Cos: their oversized fit isn't a mistake or inconsistency—it's intentional design. This Swedish brand (part of the H&M Group) is all about that minimalist Scandinavian aesthetic where clothes have a relaxed, architectural quality. Boxy silhouettes, dropped shoulders, and plenty of room are central to their design ethos.

I've been shopping at Cos for about six years now, initially attracted by their beautiful quality basics and timeless designs. But it took me three returns and several changing room disasters before I cracked their sizing code. I'm a consistent size 12 across most British high street brands, but in Cos I range from a size 8 to a size 10 depending on how fitted I want the piece to feel.

The consistent feedback from Cos customers? "Runs large," "size down," "much bigger than expected." This isn't a flaw—it's their aesthetic. But if you're not prepared for it, you'll end up with clothes that overwhelm your frame rather than complement it.

Breaking Down Cos Sizing By Category

Let me give you specific guidance based on years of trial and error:

Shirts and blouses – Size down one to two sizes. Even their "fitted" shirts are cut quite generously. If you want a relaxed but not overwhelming fit, size down once. For a more tailored look, go down two sizes.
Knitwear and jumpers – Size down at least once, possibly twice. Their knitwear is notoriously generous. What's labelled "relaxed fit" is genuinely oversized. Their "oversized" styles are absolutely enormous.
Dresses – Depends on the style. Shirt dresses and relaxed styles: size down once. Fitted or structured dresses: true to size. Always check the specific measurements.
Trousers – Generally true to size, though their wide-leg styles are very wide. If you want a more contemporary wide-leg rather than dramatically voluminous, consider sizing down.
Coats and jackets – Size down one size unless you plan to layer chunky knitwear underneath. Their coats are cut with plenty of room.
T-shirts and basics – Size down one size minimum. Their basic tees are cut quite boxy and long in the body.
Tailored pieces – More likely to be true to size, but still check measurements. Their blazers often have dropped shoulders even in "standard" fits.

Why Cos Sizing Feels So Different

Cos is a Swedish brand, and Scandinavian sizing tends to run larger than UK sizing anyway. But beyond that, their design philosophy celebrates volume, negative space, and architectural shapes. They're not trying to create clothes that hug your curves—they're creating pieces with interesting silhouettes and proportions.

I once asked a Cos sales assistant about their sizing, and she explained that many of their pieces are designed to be "one size fits many"—deliberately generous so a size 10 could fit both a size 8 and a size 12 comfortably. It's about versatility and that relaxed Scandi aesthetic, not about traditional UK sizing standards.

How Cos Compares to Other Minimalist Brands

If you shop at other Scandinavian or minimalist brands, here's how Cos sizing compares:

Arket – Also Swedish, also part of H&M Group, but runs slightly smaller than Cos. If you're a size 10 in Arket, you'll likely be a size 8 in Cos. Arket's fits are more true to standard sizing whilst still maintaining that Scandi aesthetic. This is my left-field recommendation for people who love Cos but want more predictable sizing.

& Other Stories – Another H&M Group Scandi brand that runs more true to size than Cos. If you're a 12 in &Other Stories, expect to be a 10 in Cos. &Other Stories has less of that deliberately oversized quality—their clothes are still beautiful and minimalist but fit more conventionally.

Monki – The youngest sibling in the H&M family, and runs smaller than Cos. Monki is more fitted and trend-led. You'll likely be one full size smaller in Monki than in Cos, possibly two sizes in knitwear. Monki is brilliant if you want that playful Scandi vibe without the oversized proportions.

Zara – Runs much smaller than Cos overall. If you're a size 12 in Zara (which itself runs small), you could easily be a size 8 or even a 6 in Cos knitwear. The sizing philosophy is completely opposite—Zara goes fitted, Cos goes relaxed.

Mango – More similar to Cos in their contemporary, minimalist aesthetic, but runs smaller. If you're a 12 in Mango, expect to be a 10 in Cos. Mango's proportions are more aligned with standard European sizing.

Massimo Dutti – Runs true to size with a more tailored European fit. You'll be 1-2 sizes smaller in Massimo Dutti than in Cos. Both brands do sophisticated minimalism, but Massimo Dutti favours structured tailoring over Cos's relaxed draping.

Whistles – British brand with a contemporary, minimalist aesthetic. Runs true to slightly small. If you're a size 12 in Whistles, you'll probably be a 10 in Cos. Whistles cuts more closely to the body than Cos's architectural shapes.

Reiss – Runs true to size with a focus on fit and tailoring. You'll typically be one size smaller in Reiss than in Cos. Reiss is all about polish and structure, whilst Cos embraces relaxed volume.

Jigsaw – True to size with a relaxed but not oversized fit. If you're a 12 in Jigsaw, expect to be a 10 in Cos. Jigsaw's relaxed tailoring is still more fitted than Cos's deliberately generous cuts.

The Petite Situation at Cos

Here's where Cos becomes particularly challenging: they don't offer a petite range, and their standard sizing runs long in addition to running large. If you're petite (under 5'4"), Cos pieces can overwhelm your frame entirely. I'm 5'5" and I find their dresses often hit me at an awkward length.

My advice for petite women? Size down even more than usual, and be prepared to get pieces tailored for length. The sleeves will likely need shortening, dresses might need hemming, and trousers will almost certainly be too long. If that sounds like too much hassle, consider Arket or & Other Stories instead—they offer similar aesthetics with more manageable proportions.

Where Cos Fits Alongside Other Brands in Your Wardrobe

If you're mixing Cos with other high street and contemporary brands, here's what to expect:

Cos vs. H&M – H&M (the parent company) runs much smaller than Cos. If you're a 12 in basic H&M pieces, you'll be a 10 or even an 8 in Cos. The quality is also noticeably different—Cos uses better fabrics and construction.

Cos vs. M&S – M&S runs true to generous UK sizing. If you're a 12 in M&S, expect to be a 10 in Cos, possibly an 8 in knitwear. The fit philosophies are completely different.

Cos vs. French Connection – French Connection runs true to size. You'll typically be one size smaller in Cos. Both do contemporary minimalism, but French Connection's fits are more body-conscious.

Cos vs. Calvin Klein – Calvin Klein runs true to American sizing (slightly generous). You'll likely be the same size or one size smaller in Cos. Both brands do clean minimalism but with different proportions.

My Top Tips for Buying Cos

After years of Cos shopping, here's what I've learned:

Always check the specific garment measurements, not just the size chart. Cos provides detailed measurements—use them religiously.
Look at the model's stats. Cos shows what size the model is wearing and their height. This is genuinely helpful for gauging fit.
Start by sizing down. If you're new to Cos, automatically order one size smaller than your usual size as your starting point.
Consider the look you want. Want that oversized Scandi aesthetic? Maybe stick with your usual size. Want something more fitted? Go down two sizes.
Pay attention to fabric. Natural fibres (cotton, linen, wool) maintain their shape. Looser weaves and jersey can stretch further with wear.
Use their generous returns policy. Cos offers free returns, so don't hesitate to order two sizes if you're uncertain.

The Smarter Way to Get Cos Sizing Right Every Time

Right, here's where I get passionate about something that's genuinely transformed my Cos shopping experience: using proper sizing tools instead of just guessing based on size labels.

I used to spend hours in Cos trying on multiple sizes of the same item, getting increasingly frustrated. Or I'd order online, get something that swamped me, send it back, reorder a smaller size, wait another week—it was exhausting. The worst part? Feeling like I didn't understand my own body because the sizing was so unpredictable.

Tellar.co.uk completely changed this. It's the UK's leading sizing tool that matches your measurements to over 1,500 brands—including Cos and all the Scandi brands I've mentioned.

Here's how it works:

Measure yourself once using your bust, waist, and hip measurements (or input a size from a brand that fits you perfectly)
Use the Store Size Lookup tool to find your precise size in Cos—it accounts for their generous sizing
Compare your Cos size to your size in H&M, Zara, Arket, or any other brand you regularly shop
It's completely free, no downloads needed, works straight in your browser

The game-changer for Cos specifically? You can see exactly how much larger they run compared to brands you already know fit you. So if you're reliably a size 12 in Whistles or Jigsaw, Tellar will show you that you need a 10 or even an 8 in Cos. No more guessing, no more oversized disappointments.

The Tellar Fashion Hub: Your Sizing Questions Answered

Beyond the sizing tool, Tellar has built this brilliant Fashion Hub—a library of free posts covering every sizing dilemma and style question. It's written by people who actually understand fashion and fit, and it's completely unbiased (no brand partnerships or sponsored content influencing recommendations).

For more help with sizing and styling, check out:

The Ultimate Clothing Sizing Guide – essential reading before buying anything online
Jeans Trends 2026 – useful if you're looking at Cos denim
Ultimate Guide to Dresses – particularly helpful given Cos's unconventional dress proportions

My Final Thoughts on Cos Sizing

To directly answer the question: no, absolutely do not size up in Cos. The brand runs large by design, and sizing up will leave you swimming in fabric. For most items, size down at least once from your usual UK high street size. For knitwear and deliberately oversized pieces, consider sizing down twice.

This isn't about vanity sizing or flattery—it's about understanding that Cos's aesthetic celebrates volume and relaxed proportions. Their size 10 is genuinely larger than a size 12 in many British brands. The key is figuring out how much of that oversized quality you actually want.

My advice? Use Tellar to check your Cos size before you buy. It takes all the guesswork out and means you'll get pieces that fit the way you want them to—whether that's their signature relaxed look or something slightly more fitted.

Have you experienced Cos's generous sizing? How many sizes do you go down? I'd love to hear whether you embrace their oversized aesthetic or prefer to size down for a more conventional fit!