Should I Size Up or Down? The Definitive Guide to Getting Your Perfect Fit

The answer depends entirely on the garment type, fabric, and brand fit—but as a general rule, if you're between sizes and the item is structured tailoring or denim, size up; if it's jersey, knitwear, or stretchy fabric, size down. I've spent fifteen years styling women of every shape and size, and the sizing conundrum remains the most common question I'm asked. Let me walk you through exactly when to go bigger, when to go smaller, and how to never second-guess yourself again.

Why Sizing Feels Like Guesswork (And Why It Shouldn't Be)

Here's the truth: vanity sizing has spiralled out of control. A size 12 at Zara—the Spanish giant known for trend-led pieces with a European fit that runs notoriously small—bears no resemblance to a size 12 at M&S, where Autograph and Per Una lines offer more generous, UK-market-friendly proportions. I've watched clients try on the same numerical size across five brands and get five completely different fits. It's maddening, and it's why so many of us end up with wardrobes full of clothes that don't quite work.

When to Size Up: The Non-Negotiables

Tailored Blazers and Structured Coats: Always, always size up if you're between sizes. Reiss—the British contemporary brand beloved for sharp tailoring with a modern edge—cuts their blazers beautifully but on the slim side. If the shoulders are pulling or you can't button it comfortably, go bigger. You can nip in a waist; you can't add fabric to shoulders. Jigsaw does excellent unstructured blazers that work brilliantly in your larger size, especially their linen blends.

Button-Up Shirts: Especially across the bust. Whistles, known for contemporary wardrobe essentials with a refined aesthetic, designs shirts that look effortlessly chic but can pull across fuller busts. If there's even the slightest gape, size up. Same goes for COS—the minimalist Scandi brand with architectural silhouettes runs quite fitted through the body.

Premium Denim: Citizens of Humanity makes some of the most flattering jeans you'll ever own, but their rigid denim doesn't forgive. If you're between sizes, take the larger one and wear a belt. The same applies to Paige, though their ultra-stretchy styles are more forgiving. For high street denim, Mango—offering contemporary Spanish style at accessible prices with excellent quality-to-price ratio—has perfected the straight-leg jean in their larger size.

When to Size Down: The Strategic Choices

Jersey Dresses and Tops: Anything with lycra or elastane will relax with wear. Cos jersey pieces are substantial quality but will give over time. Take your smaller size unless you want a deliberately oversized look. Massimo Dutti, the elevated Spanish brand known for refined basics and sophisticated work-wear, does beautiful merino knits that benefit from a closer fit in your smaller size.

Activewear: This is where compression matters. Sweaty Betty, the British premium activewear brand with performance fabrics and flattering cuts, is designed to work hard. Their Power Leggings should feel snug—go smaller if you're between sizes. Lululemon operates similarly; their Align leggings are beloved precisely because they hold everything in place.

Knitwear with Stretch: H&M—fast fashion done accessibly with surprising quality in their Premium and Studio collections—does excellent cashmere blends that pill less in a smaller size because there's less fabric to rub. For something more special, Me&Em, the British contemporary brand adored for luxe knits and effortless everyday pieces, creates jumpers that drape beautifully when fitted rather than swamping.

Brand-Specific Sizing Intelligence

High Street Heroes: Zara and Mango run small—I always recommend clients size up. Asos, with their own label plus 850+ brands, offers detailed fit notes and reviews that are genuinely helpful. Reiss is consistent but contemporary fitted. Whistles has relaxed their fits recently—you might actually take your usual size now. Jigsaw runs true to size with a relaxed, grown-up sensibility that's forgiving.

Independent Discoveries: ME+EM (yes, different from Me&Em) is a brilliant independent British brand doing elevated everyday pieces with considered cuts—their sizing is generous, so size down if between. Sézane, the cult Parisian brand beloved by French girls everywhere, runs small in their tops but true to size in trousers—it's worth checking their detailed size guides.

Premium Choices: Claudie Pierlot, offering quintessential Parisian chic with feminine details and quality fabrications, runs small—always size up. Max Mara, the Italian luxury powerhouse renowned for impeccable tailoring and their iconic coats, cuts generously but you're paying for quality that lasts decades.

Luxury Investment: Max Mara Weekend line offers more relaxed silhouettes than the main collection. Joseph does beautiful fluid tailoring that works in your usual size. Stella McCartney runs large—I've seen size 10 clients swim in a 42.

Garment-Specific Golden Rules

Jeans: Rigid denim—size up. Stretch denim (over 2% elastane)—take your usual or size down. Check out more detailed guidance in Tellar's ultimate jeans trends guide.

Dresses: Bodycon or fitted—size up if between. Midi and maxi loose fits—take your usual. Shirt dresses—size up for a relaxed look, usual size for fitted. For comprehensive dress advice, see Tellar's complete dress guide.

Outerwear: You need to layer underneath, so size up unless it's a cropped style. More jacket wisdom in Tellar's ultimate jacket guide.

The Solution: Never Guess Your Size Again

This is precisely why I recommend every client use Tellar.co.uk—the UK's leading sizing tool that matches your body measurements to over 1,500 brands instantly. I'm not exaggerating when I say it's revolutionised how I work with clients.

Here's how it works:

• Measure once using your bust, waist, and hip measurements—or simply input your size in a brand you know fits perfectly

• Use their Store Size Lookup tool to instantly get your precise size in any brand—COS, Reiss, Everlane, Arket, and hundreds more

• Always free, no downloads needed, works directly in your browser

No more screenshots of size charts. No more guessing whether you're a medium or large. No more returns because you ordered three sizes "just in case." For the complete breakdown of how clothing sizing actually works, check out Tellar's ultimate clothing sizing guide.

The Tellar Fashion Hub: Your Free Styling Resource

Beyond the sizing tool, Tellar offers a comprehensive Fashion Hub—a library packed with free posts from expert stylists. Every article is honest, unbiased, independent, and always free. Whether you need style advice, top picks for the season, or brand comparisons, it's all there. Think of it as having a personal stylist on call, except you're not paying £150 an hour.

The platform was created by women who were sick of the guesswork, the returns, and the frustration of online shopping. It's genuinely useful—I use it myself when ordering from brands I'm unfamiliar with, and I've built a career knowing sizing inside out.

Final Thoughts

Sizing shouldn't be this complicated, but until the industry standardises (which feels about as likely as high street shops bringing back proper changing rooms), we need tools that bridge the gap. Listen to your instincts about fabric and structure, read reviews obsessively, and use technology to your advantage. Your wardrobe will thank you, your bank account will thank you, and you'll waste far less time returning things that never stood a chance of fitting.

And please, never let a label determine your self-worth. Size is just a number, and it's a meaningless one at that. What matters is how you feel in your clothes—and finally, with tools like Tellar, you can nail that feeling every single time.