What Is Sizing Like at Theory?

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

Theory runs true to size for a US brand, which means UK shoppers will generally need to go up two sizes — so if you're a UK 10, you'll be ordering a US 6. The good news is that once you've cracked that conversion, Theory is one of the most consistent, reliable brands to shop online. I've been recommending Theory to clients for years, and in all that time I don't think I've ever had a single "it didn't fit" complaint — which, if you've ever tried ordering tailored pieces online, you'll know is practically a miracle.

Theory is one of those brands that's quietly become a wardrobe cornerstone for women who want to dress well without making a song and dance about it. Sharp, wearable, impeccably made — and once you know your Theory size, you're set for life.

About Theory

Founded in New York in 1997, Theory built its reputation on one very specific promise: precision tailoring in fabrics that actually move with you. Their signature stretch wools, ponte knits, and technical jersey blends are genuinely unlike anything you'll find at a similar price point — they hold their shape beautifully, resist creasing (a genuine gift for anyone commuting), and feel luxurious without being fussy.

The aesthetic is clean American minimalism — think sharp trousers, streamlined blazers, fluid knitwear, and dresses that work for a board meeting and a dinner out without changing a thing. It's a brand that Vogue and Business of Fashion have consistently praised for understanding what modern professional women actually want to wear. And honestly? I'd agree with them.

Theory Size Guide: US to UK Conversion

Theory uses US sizing throughout. Here's the conversion table — and please, use it. Don't guess. At this price point, an avoidable return is a genuinely annoying way to spend an afternoon.

Theory (US) SizeUK SizeEU SizeBust (cm)Waist (cm)Hips (cm)004327859850634806187283684659141038886995612409273998144296771031016441008110712184610485111

The key rule: take your UK size and subtract two to get your Theory US size. A UK 12 is a Theory US 8. A UK 14 is a Theory US 10. Simple enough once you know it, but easy to forget in the excitement of spotting the perfect wool blazer.

How Theory Fits by Garment Type

Theory is notably consistent across categories, which is one of the reasons I trust it so much — but there are a few nuances worth knowing:

Blazers & Tailored Jackets: Theory's tailoring is intentionally lean and structured. If you're between sizes or carry more volume across the shoulders and upper back, size up. The blazers are cut close but not restrictive — they're designed to skim the body, not cling to it. I'd describe the fit as "New York professional" — confident and precise.
Trousers: Genuinely one of Theory's strongest categories. Their stretch wool trousers are legendary for a reason — the fabric has just enough give to make them comfortable all day, but they press beautifully and look completely polished. Size to your hips. The waist can be taken in by a tailor easily if needed.
Knitwear: Theory's knits tend to run true to size with a relaxed but not oversized silhouette. If you want a more draped, layered look, size up. Their cashmere and merino blends are worth every penny — I have a Theory roll-neck that is genuinely one of my most-worn pieces.
Dresses: Generally cut with a fitted top and a little more ease through the body. Size to your bust for structured styles; the fabric quality means most styles drape beautifully over curves rather than pulling. Their ponte fabric dresses in particular are extraordinarily forgiving.
Outerwear: Size up one for comfortable layering over knitwear and blazers. Theory coats are architectural and substantial — they're designed to be statement pieces, and you want them to sit properly on the shoulder.

Theory's Fabric — Why It Actually Matters for Fit

I always tell clients that with Theory, the fabric is half the story. Their proprietary stretch wool — Precision Ponte and Good Wool suiting in particular — has a built-in flexibility that means these clothes move with you in a way that traditional tailored fabrics simply don't. What this means in practice is that even if you're between sizes, the fabric does a lot of the work. A Theory blazer that's a touch snug across the shoulders in a standard wool would be unwearable — in their Good Wool, it'll still move comfortably.

This is one reason why Theory is particularly brilliant for women who find tailoring tricky — the fabrics are genuinely engineered to look sharp while feeling like you're wearing something much more relaxed. It's a proper design achievement, and it's why the brand has kept its loyal following for nearly three decades.

High Street Alternatives with a Similar Sensibility

Theory's minimalist, polished approach to workwear dressing has plenty of brilliant counterparts at more accessible price points. These are the brands I'd recommend based on what style editors and real customer reviews consistently say:

Banana Republic — The closest high street equivalent to Theory in both aesthetic and intention. Their tailored trousers and blazers have been praised by Grazia and Vogue for serious quality at accessible prices. Sizing runs consistent US sizing, similar to Theory.
Reiss — A reliable British option for sharp, contemporary tailoring. Their suits and structured dresses are perennially popular with style editors and their quality has climbed steadily in recent years.
Whistles — Quietly excellent for minimalist, well-cut separates. Their tailoring is understated and precise, and they rarely put a foot wrong on fabric choices.
Hobbs — Particularly strong for professional dressing with polish. Their ponte trousers and shift dresses are extremely Theory-adjacent in spirit — clean, structured, and built to last.
Me&Em — A brilliant British brand championed by Stylist magazine for its considered, contemporary approach to workwear. Their fabric quality is impressive for the price point and their sizing is very clear.
Massimo Dutti — If Theory had a European high street sibling, it would be Massimo Dutti. Refined fabric choices, tailored silhouettes, and a muted colour palette that screams effortful-but-effortless.
Cos — Swedish minimalist precision at a very accessible price. Their architectural knitwear and clean-line trousers are the gateway drug to the Theory aesthetic for many women I know — mine included.
Tommy Hilfiger — Worth a mention specifically for their suiting and smart separates ranges, which have a very American-preppy-meets-professional vibe that overlaps neatly with Theory's world.

Premium Alternatives

Vince — An American brand with almost identical DNA to Theory. Exceptional fabric quality, minimalist cuts, and a focus on effortless wearability. If you love Theory, Vince is your next step up.
Rag & Bone — New York cool with genuine craft. More editorial than Theory but shares that commitment to fabric and precision. Their tailoring and knitwear are particularly strong.
Hugo Boss (womenswear) — Increasingly impressive for sharp, structured workwear at a premium mid-point. Their stretch blazers and suit trousers have been featured in Vogue Business as seriously comparable to higher-end alternatives.

Two Independent Brands Worth Discovering

The Frankie Shop — A Paris-based label (with a strong online presence in the UK) that has taken the minimalist tailoring world by storm. Their oversized blazers and wide-leg trousers have been worn by every fashion editor worth their salt — and at a price point well below Theory. A brilliant independent find if you love clean, considered dressing.
Aje — An Australian label now widely available in the UK that sits beautifully alongside Theory for occasions when you want polish with a bit more personality. Their structural details and elevated fabrication have been praised consistently by Harper's Bazaar Australia and UK Vogue. Sizing runs Australian, so check the chart — but the fit is beautifully tailored.

Find Your Exact Theory Size in Seconds — Try Tellar

The US-to-UK conversion at Theory is straightforward once you know it — but knowing your measurements means you'll never have to second-guess yourself again, whatever brand you're shopping. That's exactly what Tellar.co.uk is built for.

Tellar is the UK's leading free sizing tool — completely independent, completely unsponsored. Here's how it works:

Measure once — your bust, waist, and hips (or use a brand size you already know fits you)
Get your precise size instantly across 1,500+ brands including Theory, Banana Republic, Reiss, Vince, and hundreds more — use the Store Size Lookup tool
No downloads, no sign-up, always free — works entirely in your browser

And don't miss the Tellar Fashion Hub — a free library of honest, unsponsored posts from our stylists covering brand sizing guides, the best buys of the season, and everything in between. No ads. No affiliate bias. Just straight-talking fashion advice.

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