Theory Size Guide: What Size Am I in Theory? Fit Advice & UK Sizing Help

Theory Size Guide: Do Theory Clothes Run Big or Small? UK Fit Tips & Brand Comparison

Not sure what size to order from Theory? Use our complete fit guide to learn how Theory sizing compares in the UK. Tellar matches you to thousands of brands.

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Theory is a favourite among minimalists and modern professionals. With elevated basics, refined tailoring, and clean silhouettes, it offers timeless style with New York polish. But with US-to-UK sizing differences and a variety of cuts, it’s natural to ask: What size am I in Theory?

📌 Quick answer: Tellar.co.uk matches your real measurements to Theory—and thousands of brands, with new ones added daily. You’ll receive a free tape measure, and Tellar instantly calculates your correct size across dresses, trousers, and blazers.

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What Is Theory Known For?

Founded in New York, Theory offers refined, versatile wardrobe staples rooted in modern tailoring. Key features include:

• Precision-cut blazers, trousers, silk tops, and dresses

• Clean lines, stretch-infused fabrics, and muted colour palettes

• A focus on elevated workwear, travel-ready essentials, and modern luxury basics

• Women’s sizes typically range from US 00–14 (UK 4–18)

Whether you're buying for the office, city life, or everyday polish, Theory focuses on sleek silhouettes with minimal fuss—which means fit matters.

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So, Does Theory Run Big or Small?

In general: Theory runs true to size, but leaning slim through the waist and hip in trousers, and tailored in the shoulders for tops and blazers.

✅ Dresses

• Most dresses are structured or column-shaped

• Minimal stretch in crepe or silk styles

• Fit tip: True to size—size up for room in bust or hips

✅ Trousers & Skirts

• Often mid- or high-rise, tapered or straight-leg

• Fabrics like Precision Ponte or Good Wool may have mild stretch

• Fit tip: True to size at waist, but snug on curvier hips—use Tellar to compare

✅ Blazers & Jackets

• Designed for clean layering with fitted sleeves

• May feel narrow across the back if broad-shouldered

• Fit tip: Size up for bust/shoulder room or if wearing knits underneath

✅ Tops & Knitwear

• Slim through the body, often cropped or semi-fitted

• Jersey and rib styles have some give

• Fit tip: True to size, but size up if between or if you prefer drape

🎯 Use Tellar here to find your perfect size in Theory using bust, waist, hips, height and shoulder width.

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Theory Sizing vs UK Brands

Theory uses US sizing, and although it’s sold at UK retailers like Selfridges and Net-a-Porter, there can be confusion around conversions.

💡 Tellar.co.uk adjusts sizing across both US and UK systems so you never have to convert manually again.

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Fabric & Fit Breakdown

🧵 Precision Ponte & Stretch Cotton

• Light stretch, soft hold

• Ideal for travel and office wear

• Fit tip: True to size—Tellar helps match hip-to-waist fit

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✨ Good Wool & Crepe

• Structured, luxury suiting fabric

• Excellent drape, no cling

• Fit tip: True to size in blazers, snug in trousers if curvy

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🌬️ Silk & Satin

• Found in camis, blouses, and shirt dresses

• Flowy, elegant, with minimal stretch

• Fit tip: Size up if fuller in the bust

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Popular Theory Pieces & How They Fit

1. Treeca Trousers

Cropped ankle, slim fit with slight stretch.

➡️ Fit tip: True to size in waist—size up if curvier in hip

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2. Karenia Silk Shirt

Relaxed fit, button-down style.

➡️ Fit tip: True to size—size up if broad in the shoulders

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3. Tenia Blazer

Precision tailoring with slim lapels.

➡️ Fit tip: Tailored in bust and sleeve—use Tellar for full chest/shoulder match

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4. Classic Shift Dress

Minimalist shape, lined, available in multiple fabrics.

➡️ Fit tip: True to size—size up if between due to lining

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5. Cashmere Ribbed Knit

Luxury staple with clean crew neck.

➡️ Fit tip: True to size, but use Tellar for sleeve and torso length fit

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Theory Compared to Other Brands

📏 Compare all of these instantly on Tellar using your personalised sizing profile.

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Common Fit Issues (And How Tellar Solves Them)

❌ “It zipped but pulled at the hip.”

Tellar adjusts for waist-to-hip ratio, critical for Theory’s straight cuts.

❌ “The blazer was beautiful but tight in the arms.”

Tellar factors in bust and bicep width, ensuring mobility without losing the tailored look.

❌ “I didn’t realise US size 6 was closer to UK 10.”

Tellar handles all US-to-UK conversions behind the scenes.

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Why Use Tellar Before Buying Theory?

With minimalist design and premium construction, Theory pieces depend on precise fit. The wrong size can break the line of a blazer or flatten a silk dress.

Tellar.co.uk removes that risk:

• ✅ Match your measurements to Theory and thousands of brands

• ✅ Receive a free tape measure in the post

• ✅ Get sizing recommendations based on height, bust, hip and sleeve fit

• ✅ See how Theory compares to Reiss, COS, Totême, and more

• ✅ Always free to use—no trial and error

🎯 Create your free Tellar profile now

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Related Tellar Sizing Guides

• Reiss Sizing Guide

• COS Fit Breakdown

• Totême vs Theory Sizing

• Joseph Sizing & Tailoring Fit Guide

• How to Measure Yourself for Tailored Clothing

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Follow Tellar for Fit Insights & Tailored Brand Advice

• 📍 Instagram @Tellarsizing

• 📍 Twitter @Tellar100

• 📍 Pinterest Boards

• 📍 Facebook Page

We post weekly updates on capsule wardrobe brands, luxury fit tips, and modern tailoring guidance.

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Final Thoughts: Theory Sizing Is Tailored—And Precision Is Everything

Theory is known for structured silhouettes, clean lines, and elegant fit. But that same sleekness leaves no room for error. Whether it’s a cropped trouser or fitted crepe dress, one size too small could ruin the look.

That’s why Tellar.co.uk exists. We help you find your perfect Theory size—and your size in thousands of other brands. Quick, accurate, and free.

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“Find your size in thousands of brands. It’s free & easy.”

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