What Is Sizing Like at On?

In short: On footwear runs largely true to size, so nine times out of ten your usual number is the right one — the brand's clothing, however, is cut to a deliberately athletic, body-skimming fit, so if you like a bit of breathing room you'll want to size up. That's the headline, but as anyone who has ever stood in a changing room clutching three sizes of the same legging knows, the detail is where it all gets interesting. I've fitted On on a lot of bodies over the years (and worn plenty of it pounding around my local park), so here's everything you actually need to know before you click "add to basket".

On trainers: how they really fit

On built its name on those instantly recognisable CloudTec soles, and the good news is that the footwear is refreshingly predictable. The standard performance range — your Cloud 6, Cloudmonster, Cloudsurfer — is designed true to size, so order your normal trainer size and you'll be grand. A few honest caveats from years of fitting feet:

Waterproof and Gore-Tex styles run a touch tighter. On themselves recommend going up roughly half a size on the waterproof range, because the membrane eats into the internal room.
They can feel a little snug against Nike or adidas. If you're swapping over from a roomier brand and you fall between sizes, go up — never down.
The Cloudmonster is the forgiving one. Its generous toe box suits wider feet beautifully, so it's my go-to recommendation for anyone who finds most trainers pinch across the forefoot.
Mind the men's-to-women's gap. If you're buying a unisex or men's colourway, women's sizing sits roughly 1.5 sizes up from men's, so a men's 6 is about a women's 7.5.

A quick confession from the fail file: I once grabbed a pair of On waterproofs in my standard size the night before a soggy weekend in the Lake District, didn't read the small print, and spent two days with my toes pressed firmly against the front. Lesson learned — with the waterproof styles, that half-size really matters.

On's clothing and activewear sizing

Here's where people get caught out. On's apparel — leggings, base layers, running tops, those lovely lightweight jackets — is cut to a proper athletic, performance-led silhouette. It's meant to sit close to the body so nothing flaps when you move, which is exactly what you want mid-run but can feel surprisingly fitted if you're used to high-street athleisure. My guidance:

Leggings and sports bras: take your true size for that compressive, supportive feel. Size up only if you actively dislike anything snug.
Tops and tees: true to size, but slim through the shoulder and waist. If you're broader up top or want a relaxed drape, go up one.
Trousers and joggers: the sweatpants in particular are cut athletic — size up if you want that off-duty, slouchy look rather than a tapered one.

My styling tips for wearing On

On has quietly become as much a lifestyle label as a running one, and I love it for that. The trick to wearing it off the treadmill is to lean into the clean, Swiss-minimal aesthetic rather than fight it: think monochrome tones, a straight-leg trouser breaking just over the trainer, and one elevated piece — a wool coat, a crisp shirt — to lift the whole thing out of "I've just been to the gym" territory. A pair of all-white Clouds with tailored trousers is one of the most quietly expensive-looking combinations going.

Where else to shop for performance and athleisure

If you're building out a sporty-but-stylish wardrobe around your On pieces, here's where I'd send you, by budget.

High street

Sweaty Betty — the gold standard for British activewear, with leggings that genuinely hold you in and a flattering, true-to-size cut.
Gymshark — brilliant value sculpting leggings and seamless sets; runs snug, so size up if you're between.
Marks & Spencer — the Goodmove range is a quiet hero for supportive, sensible kit that won't break the bank.
Uniqlo — unbeatable for technical base layers and AIRism tees to wear under your On jackets.
Superdry — sporty hoodies and joggers with that easy off-duty energy.
Next — a reliable, well-priced activewear offering that's grown up enormously.
New Look — the place for trend-led athleisure basics without the commitment.

Premium

Lululemon — the cult buttery Align legging and an athletic cut that mirrors On's own slim silhouette beautifully.
Alo Yoga — LA-cool studio-to-street pieces; sizing runs small, so order up.
The North Face — for technical outerwear that bridges trail and city, sitting right alongside On's performance ethos.

Luxury and designer

Ralph Lauren — the RLX performance line marries proper sportswear engineering with that polished, preppy finish.
Max Mara — the Leisure line proves elevated, sumptuous loungewear can still feel sporty; worth knowing On itself has flirted with the luxury world via its Loewe collaboration.

Two to know (my left-field picks)

SOAR Running — a cult London label making seriously considered, beautifully engineered running kit for people who take their miles personally. Quietly the connoisseur's choice.
Bandit Running — a Brooklyn indie with a cool, community-driven attitude and genuinely thoughtful, gender-considered fit. The brand to drop into conversation before everyone else catches on.

How Tellar takes the guesswork out

Honestly, the whole reason I built Tellar is so you never have to gamble on a size again. Tellar is the UK's leading sizing tool — you measure once using your bust, waist and hip, or simply tell us a brand size you already own, and we instantly match your body to over 1,500 brands. No more squinting at size guides, no more three-of-everything orders clogging up your returns pile.

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

And when you want the styling alongside the science, the Tellar Fashion Hub is a growing library of free posts from our stylists — honest, unbiased, independent and always free. Style advice, top picks and the best brands, all in one place.

Find your exact On size in seconds

Stop guessing and start shopping with confidence. Match your body to 1,500+ brands instantly — free, no downloads, no fuss.

Visit Tellar Try the Store Size Lookup

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Happy running — and happy shopping,Ella BlakeSenior Fashion Stylist, Tellar