Under Armour Sizing: Does It Run Small, Large or True to Size?

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

Under Armour runs true to size for most women's pieces, but with a noticeably more compressive, athletic fit than everyday sportswear — so if you're between sizes or prefer a relaxed feel, sizing up is often the smarter call. I've had my own share of Under Armour moments — a sports bra that fitted perfectly on paper but felt like a rib cage, and leggings that I absolutely love once I went up a size. Once you know how their sizing works, it genuinely clicks into place.

Understanding the Under Armour Fit Philosophy

Under Armour is a performance brand first, fashion brand second. Their fits are designed for training, meaning compression, support and a close body-skimming silhouette are baked into the design. This isn't a flaw — it's intentional. But it does mean:

If you're used to more relaxed activewear (think oversized gym sets from the high street), Under Armour will feel tighter straight away.
The brand uses US sizing, which broadly aligns with UK sizing — but there are a few nuances worth knowing before you click buy.
Compression garments like HeatGear leggings and sports bras are specifically designed to feel snug. That's the point. But it also means your usual size might feel like a size too small at first wear.

Under Armour Women's Size Guide

Under Armour uses a standard XS–XXL letter sizing system for most women's pieces, with some numeric sizing for specific categories like shorts and waistbands. Here's how their sizing maps across UK, US and EU:

UA SizeUK SizeUS SizeEU SizeBust (cm)Waist (cm)Hips (cm)XS6–82–432–3480–8563–6787–91S8–104–634–3686–9068–7292–96M10–128–1038–4091–9673–7797–102L12–1410–1240–4297–10278–83103–108XL14–1612–1442–44103–10984–90109–115XXL16–1816–1846–48110–11791–97116–122

My tip: Always go by the measurement chart rather than defaulting to your usual letter size. Under Armour's XS and S in particular can come up smaller than you'd expect compared to British high street sizing.

Leggings & Bottoms — The Real-World Fit

Under Armour leggings are genuinely excellent — high waistband, great compression, moisture-wicking fabric that actually works in a sweaty spin class. But here's what I've noticed:

HeatGear leggings stretch generously and tend to feel true to size once you've worn them for a few minutes — the fabric relaxes slightly with body heat.
ColdGear bottoms have more structure and less stretch. I'd suggest going up one size here, especially if you're on the higher end of a size range.
The waistband sits high and holds well — great for anyone who doesn't want things migrating mid-workout. If you carry weight around the waist, size up for comfort rather than compression.
Shorter inseam options are available in some ranges — worth checking if you're petite, as the standard leg length can be quite long.

Sports Bras — Go Up at Least One Size

I'll be honest — this is where most women come unstuck with Under Armour. Their sports bras are engineered for maximum support, which means the band fits tight and the cups run small. If you're normally a 34B, don't be surprised if a 36B in Under Armour feels more comfortable. My advice:

Size up from your usual sports bra size as a starting point.
High-impact styles (like the Infinity or Armour Underwire) run especially small — I'd recommend going up a full cup size and potentially a band size too.
Medium-impact and low-impact bras (like the Seamless) are more forgiving and closer to a standard fit.

Tops & Jackets — Depends on Your Fit Preference

Under Armour's compression tops run true to size but feel tight — that's intentional for training. Their looser, woven jackets and studio-style tops are more relaxed and typically fit as you'd expect from a UK size. A few things to bear in mind:

If you're buying a running top for performance — size true to get the compression benefits.
If you're buying a zip-up or training jacket for the gym floor or casual wear — true to size works well.
UA's Unstoppable range (more streetwear-leaning pieces) fits more generously and is great if you want a slightly oversized feel.

Where Else to Shop for Women's Activewear

Under Armour is brilliant for performance, but it's definitely not the only game in town. Here's where I'd direct my clients depending on their style, budget, and fit preferences:

High Street & Accessible

Sweaty Betty — A British favourite for a reason. Excellent leggings with a great waistband, and sizing is generally more generous than UA. Brilliant for yoga and Pilates as well as HIIT.
Lululemon — Premium pricing but genuinely worth it for the quality and longevity. Align leggings are the benchmark for many women. Sizes tend to run true with a flattering, sculpted fit.
Gymshark — Hugely popular and much more affordable. The Vital Seamless range is a particular favourite — flattering, supportive and great sizing. Good for curvier frames.
ASOS — Their own activewear range and curated third-party brands mean a huge size range and options from XS to XXXXL. Great for budget-friendly basics.
H&M Move — Genuinely impressive for the price. Their Move leggings and sports bras are well-reviewed and run fairly true to size across the range.
Superdry Sport — Underrated for activewear. Good quality technical fabrics and a slightly more relaxed fit than performance-only brands. Sizing is generous and consistent.
Abercrombie & Fitch — Their YPB (Your Personal Best) activewear line has had genuinely glowing reviews. Soft, high-waisted and flattering across different body shapes.

Independent & Under-the-Radar Picks

Varley — A London-founded activewear brand with a beautiful aesthetic and thoughtful sizing. Less compression than UA, more lifestyle-leaning. Their bra sizing guide is excellent and reliable.
Girlfriend Collective — Made from recycled water bottles (yes, really), with one of the most genuinely inclusive size ranges in activewear — XS through 6XL. The leggings are soft, compressive and run true to size. Brilliant brand if sustainability matters to you.

Stop Guessing Your Size. Use Tellar — It's Free.

Under Armour's sizing quirks are exactly why Tellar.co.uk exists. It's the UK's leading clothing sizing tool — matching your exact measurements to 1,500+ brands instantly, so you never have to read another size guide again.

✔ Measure once using bust, waist, hip or your existing brand size
✔ Use the Store Size Lookup tool to get your precise size in any brand — Under Armour, COS, Reiss, Lululemon & more
✔ Always free. No downloads. Works in-browser.

Plus, the Tellar Fashion Hub is packed with free styling posts from our top stylists — honest, unbiased and independent. Style advice, top picks, best brands — all in one place.

Find My Size at Under Armour →