What Is Sizing Like at Onia Clothing?

By Ella Blake — Fashion Stylist | Tellar Fashion Hub | Always honest, unbiased & unsponsored

Onia runs small and is sized in US measurements — so as a UK shopper, you'll almost always need to size up by one, sometimes two sizes, and cross-reference their size chart carefully before ordering. It's one of those brands where understanding the conversion genuinely makes the difference between a swimsuit that looks incredible and one that ends up straight back in the post.

I first came across Onia a few years ago when a client came back from a trip to New York raving about a swimsuit she'd picked up. She held it up and I immediately understood the fuss — that particular shade of terracotta, the fabric that somehow looked both relaxed and elevated at the same time, and a cut that was quietly confident rather than look-at-me. She'd sized up one from her usual US size, and it fitted her perfectly. I went home and ordered one that same evening. I did not size up. Reader, it went straight back.

Since then I've got very familiar with how Onia fits, and I'm going to save you the same expensive mistake.

The US Sizing Issue — What You Need to Know First

Onia is a New York brand through and through — the whole aesthetic is that effortless, sun-drenched American resort luxury. But this means they size in US measurements, and UK shoppers consistently catch themselves out. The basic rule is simple:

UK sizing runs two sizes ahead of US sizing. So a UK 10 = US 6, a UK 12 = US 8, and so on.
Onia also cuts slim. Their aesthetic is lean and minimal — the brand is built around a particular kind of pared-back, body-conscious silhouette. This is not a relaxed, forgiving fit. On top of the US/UK conversion, many customers (including me) find sizing up one US size gives a more comfortable, flattering result.
Swimwear vs. ready-to-wear: Their swimwear runs noticeably smaller than their clothing. For their linen trousers, shirts, and dresses, the US-to-UK conversion alone usually does the job. For swimwear, plan to go up at least one US size from your converted size.

Ella's Quick Rule: Find your UK size. Convert to US (subtract 4 from your UK number). Then go up one US size for swimwear, or stay at your converted size for clothing. When in doubt, size up — Onia's fabrication drapes beautifully and a slightly looser fit still looks intentional.

Onia Women's Size Conversion Chart

Here's a full conversion table to take the guesswork out of it:

Important: Onia's own website gives detailed measurements per style — always check the individual product's size guide rather than relying purely on this conversion, particularly for swimwear styles with structured cups or underwire, where the fit is more technical.

How Different Onia Pieces Fit

Onia spans swimwear, resort clothing, and some casual everyday pieces. The fit varies meaningfully between categories, so let me break it down:

One-piece swimsuits: This is Onia's heartland and where sizing matters most. They are cut lean through the torso — the whole silhouette is designed to elongate. If you are anything other than very slim through the middle, size up one US size from your standard converted size. The fabric has enough stretch to accommodate, and a slightly larger size won't look baggy — it'll look right.
Bikini tops: Onia offers both underwired and soft-cup styles. Underwired styles run closest to true size — these are more structured so there's less room for error. Soft-cup and bandeau styles have more stretch and forgiveness; I'd still suggest sizing up one if you're between sizes.
Bikini bottoms: True to US size once converted, generally speaking. The cut is minimal — high-leg cuts in particular show a lot of hip, which is gorgeous on the right body but worth knowing in advance.
Linen and cotton clothing: Onia's resort wear range — their wide-leg linen trousers, shirts, and dresses — is genuinely beautiful, and here the sizing is more relaxed. True to your converted US size in most cases. The relaxed cuts are very forgiving.
Cover-ups and kaftans: These are designed to be worn loose, so sizing up one isn't necessary — the drape is built in. True to size or even size down if you want something more fitted.
Lounge and jersey pieces: Cut with some stretch and drape. True to size once converted from UK to US, with no need to size up.

What Makes Onia Worth the Investment?

I want to make a case for Onia here, because the price point — swimwear starting around £150–£200 — makes a lot of people hesitate. Having bought and worn their pieces over several seasons now, I'd say the quality is genuinely exceptional. The fabric — particularly their signature textured crinkle material and their linen blends — washes and wears beautifully without pilling, fading, or losing shape. These are pieces that look just as good in year three as they did on day one.

Stylistically, it sits in a very particular sweet spot: it's not trying too hard, it's not logo-heavy, it's not screaming for attention. It's the sort of thing you wear on a holiday in Positano and look like you own the place. If that sounds like your aesthetic, it's worth every penny.

The colour palette tends to run in beautiful, muted, sun-bleached tones — terracottas, soft greens, faded blues — with occasional bolder moments. They collaborate well with other brands and designers, and the collaborative capsule collections are often where the most interesting pieces show up. Worth keeping an eye on their drops.

Shopping Tips for UK Buyers

Order from Onia's own website — they ship to the UK and returns are manageable. UK stockists are limited, which makes getting the size right before ordering all the more important.
Check Net-a-Porter and Matches Fashion for Onia — both carry a curated selection and have excellent UK returns policies, which takes some of the sizing risk away.
Measure your bust and waist before ordering swimwear — don't rely on your dress size alone. Onia's own measurement guide is detailed and actually useful.
End-of-season sales are worth watching — prices drop significantly in late August and February, and quality pieces at sale price are a genuinely brilliant investment.

Brilliant Alternatives If Onia Isn't Quite the Right Fit

Whether the price point isn't right, the fit isn't working, or you just want to explore the same elevated resort aesthetic at different budgets — here are my honest picks:

🛍 HIGH STREET & ACCESSIBLE

Mango — Consistently one of the best high street options for elevated swimwear and resort wear. Their linen pieces and swimsuits capture a genuinely Onia-ish aesthetic at a fraction of the price. Sizing is broadly true to size in EU/UK.
Zara — Has stepped up its swimwear game significantly in recent seasons. The one-piece styles in particular can be genuinely beautiful. Sizes run small — as with Onia, sizing up one is often wise.
Anthropologie — Brilliant for the boho-luxe resort aesthetic. Their swimwear collaborations and own-brand pieces have real individuality, and the quality is a step above most high street. True to size.
COS — For the minimalist, pared-back end of the Onia aesthetic. Their swimwear and resort-ready separates are beautifully simple and very well made for the price. True to size, slightly relaxed fit.
& Other Stories — Stunning swimwear ranges each season that punch well above their price point. Strong on colour and print in a way that feels considered rather than chaotic. True to size.
Me&Em — Brilliant for the resort clothing side — their linen trousers, easy shirts, and sundresses have that quiet luxury quality at a much more accessible price. True to UK size throughout.
Boden — Seriously underrated for swimwear. Brilliant fit across a wide size range, excellent sun-protective options, and a colour story that's always lovely. True to UK size.

✨ PREMIUM STEP-UP

Reiss — For resort-ready dressing beyond the pool — beautiful linen dresses, tailored shorts, and easy co-ords with that same understated luxury feel. True to UK sizing.
Calvin Klein — Their swimwear range is the closest thing to Onia at a more accessible premium price point. Clean lines, excellent quality fabrics, and a minimalist aesthetic that travels beautifully. Runs slightly small — size up one.

💎 INVESTMENT & DESIGNER

Vilebrequin — The French Riviera equivalent of Onia. Exceptional quality, distinctive prints, and a heritage that speaks for itself. European sizing — check their guide.
Max Mara — For the most elevated resort-wear wardrobe building beyond swimwear — linen blazers, silk co-ords, and the kind of pieces that look brilliant beside a pool or at dinner. True to European sizing.

🌿 INDEPENDENT PICKS YOU MIGHT NOT KNOW

Hunza G — If you haven't already fallen down this rabbit hole, allow me to introduce you. This British brand makes the most flattering swimwear I've ever worn, full stop. The signature crinkle nylon fabric has zero-lycra stretch that somehow fits every body shape beautifully. One size fits most styles — genuinely. It's the brand I recommend more than any other when clients say swimwear shopping makes them miserable. Worn by everyone from supermodels to the rest of us, and the price is remarkably reasonable for the quality.
Heidi Klein — A British luxury beachwear brand that deserves to be far better known outside of the fashion world. Exceptional quality swimwear and resort wear, cut with real thought for women's bodies, and a beautiful colour palette. The sizing is true to UK sizing — refreshingly straightforward — and the pieces last for years. If you love Onia's aesthetic but want something with a slightly more generous, wearable fit, this is your answer.

TELLAR.CO.UK — THE UK'S LEADING SIZING TOOL

Never Get Onia's Sizing Wrong Again

Tellar instantly matches your exact measurements to 1,500+ brands — including US brands like Onia where the conversion trips everyone up. Free, in-browser, no downloads needed.

Find My Onia Size →Explore Tellar.co.uk

This is exactly the kind of sizing headache that Tellar was built to solve. You measure once — bust, waist, hip, or simply enter a size you already know works — and Tellar cross-references your measurements against 1,500+ brands instantly. For a US brand like Onia, where the conversion plus the brand's own sizing quirks stack up, having a tool that cuts through all of that is genuinely invaluable. It's completely free, works in your browser, and the Tellar Fashion Hub has hundreds of brand sizing guides, style posts, and honest fashion advice — all independent, all unsponsored.