The Hidden Science Behind Fashion Sizing: Why Your Size Varies and How to Get It Right Every Time

By Ella Blake, Technical Fashion Stylist15 years of industry experience in fit technology and garment constructionLast Updated: October 2025Prepared ad draft submission for: The Time

Introduction: The £7 Billion Problem

www.Tellar.co.uk

The average UK consumer returns one in every three online fashion purchases, with incorrect sizing accounting for approximately 60% of these returns. This translates to over £7 billion in annual costs for UK retailers alone, not to mention the environmental impact of reverse logistics and the frustration experienced by millions of shoppers.

After fifteen years working as a technical fashion stylist across high street and luxury brands, I've witnessed firsthand how sizing inconsistency has evolved from a minor inconvenience into one of the fashion industry's most pressing challenges. The question isn't whether you've ordered the wrong size online—it's how many times it's happened this month.

The Anatomy of Size Inconsistency: Why Your Size Isn't Really Your Size

Vanity Sizing and Brand Positioning

The fashion industry's relationship with sizing standards has always been complicated. Unlike the metric system used in manufacturing or the regulated sizing in children's clothing, adult fashion sizing remains largely unregulated across most markets.

Vanity sizing—the practice of labelling garments with smaller size numbers than their actual measurements—has proliferated since the 1980s. Research from the London College of Fashion indicates that a UK size 12 dress today measures approximately 5cm larger across the bust than a size 12 from 1980. Brands implement this strategy to make customers feel better about their purchases, capitalising on the psychological satisfaction of fitting into a smaller size.

However, the practice creates significant downstream problems. When a shopper discovers they're a size 10 at Zara but a size 14 at Boden, the cognitive dissonance creates purchasing anxiety. My work with pattern grading teams has shown me that this isn't accidental—it's a deliberate brand positioning tool.

Target Demographics and Fit Models

Every fashion brand builds its sizing architecture around a "fit model"—a person whose measurements represent the brand's target customer. Here's where it gets complicated: these fit models vary dramatically between brands.

High street brands typically use fit models with measurements representing their core demographic:

Zara targets a younger, more fashion-forward customer (often using a model closer to industry sample size 8-10)
Marks & Spencer caters to a broader age range with a more generous cut (fit model typically size 12-14)
ASOS, as a marketplace, hosts dozens of brands with completely different fit philosophies

Luxury brands often maintain smaller size runs, with their largest sizes equivalent to high street medium sizes. During my time consulting for a British heritage brand, their size 14 measured identically to a high street size 10—a deliberate choice to maintain exclusivity.

This disparity extends beyond vanity. Different demographics have genuinely different body proportions. A 25-year-old and a 55-year-old with identical bust, waist, and hip measurements will carry weight differently, requiring different pattern adjustments for optimal fit.

Manufacturing Tolerances and Production Realities

Even within a single brand, sizing varies batch to batch. Garment manufacturing operates within "tolerance ranges"—acceptable measurement deviations from the technical specification.

Industry standard tolerances typically allow:

±1.5cm on body measurements
±2cm on length measurements
±0.5cm on critical areas like sleeve heads

When I conduct fit sessions, I regularly see variations of 2-3cm between identical garments from the same production run. Multiply this across different factories, fabric batches, and production seasons, and you understand why two "identical" size 12 dresses can fit completely differently.

Fabric composition compounds this issue. A 100% cotton shirt fits differently than a cotton-elastane blend, even in the same size. Stretch fabrics mask poor fit, leading brands to use tighter base patterns, knowing the fabric will accommodate body variation.

Regional and International Sizing Systems

The global fashion supply chain creates additional complexity. UK sizes, US sizes, European sizes, and Japanese sizes all operate on different numbering systems:

UK 12 = US 8 = EU 40 = Japan 11
UK 16 = US 12 = EU 44 = Japan 15

However, these conversions aren't standardised. An American brand might use US sizing but cut garments for American body proportions, which differ from UK proportions. According to data from the British Retail Consortium, the average UK woman is shorter and has a different torso-to-leg ratio than her American counterpart.

When brands expand internationally, they rarely adjust patterns for regional body differences. Instead, they simply convert the size label, creating fit issues for local customers.

The Rise and Fall of Standardisation Attempts

The EN 13402 Standard: A European Ideal

The European Union introduced the EN 13402 standard in 2007, attempting to create a measurement-based sizing system across member states. Rather than arbitrary numbers, garments would be labelled with actual body measurements: bust, waist, hips, and height.

In theory, brilliant. In practice, almost completely ignored.

The fashion industry resisted implementation for several reasons:

Cost: Relabelling inventory and updating systems required significant investment
Brand identity: Luxury brands didn't want to abandon their established sizing hierarchies
Consumer confusion: Shoppers were accustomed to size numbers, not centimetre measurements
Vanity sizing benefits: Brands profited from the psychological advantages of size deflation

Today, you'll occasionally see EN 13402 measurements on labels alongside traditional sizes, but adoption remains minimal outside Germany and Austria.

ASTM and ISO Standards: Industry Guidelines Without Teeth

In the US, ASTM International published sizing standards (particularly ASTM D5585 for women's wear). The International Organization for Standardization (ISO) developed ISO 8559, covering garment measurement techniques. Both remain voluntary guidelines rather than enforceable regulations.

During my consulting work with pattern cutting departments, these standards serve as reference points, but brands customarily deviate based on their target market. A size 10 might follow ASTM guidelines at one retailer and be 5cm larger at another—both technically "correct" according to their internal specifications.

The Technology Gap: Why Existing Solutions Fall Short

Recognising the sizing crisis, numerous technology companies have attempted to solve the problem over the past decade. However, most solutions suffer from fundamental limitations that prevent mainstream adoption.

The B2B Focus Problem

The majority of sizing technology platforms—tools like True Fit, Fit Analytics, and others—are built primarily for retailers rather than consumers. These enterprise solutions integrate into brand websites, but the consumer experience remains fragmented:

Multiple profile creations: A shopper must create separate size profiles for each retailer using the technology
Limited brand coverage: Even major platforms cover only a fraction of where consumers actually shop
Retailer dependency: If a brand doesn't subscribe to the service, consumers receive no assistance
Data silos: Size information learned on one platform doesn't transfer to others

From a retailer perspective, these tools provide valuable analytics about return rates and sizing trends. For consumers, however, the experience remains disjointed. After years of working with both retailers and customers, I've observed that fragmented solutions ultimately serve neither party optimally.

The Static Size Chart Problem

Many websites simply digitise size charts without adding intelligence:

Image-based charts: Some sites merely photograph size charts, providing no interactive functionality or personalisation
One-size-fits-nobody recommendations: Basic tools ask for one brand size and extrapolate to others without accounting for style differences, fabric stretch, or cut variations
No measurement validation: Systems rarely verify that user-provided measurements are accurate or even physically possible

I've personally witnessed shoppers input measurements that would require anatomical impossibilities (such as a 90cm bust with a 100cm waist), yet systems provide recommendations without question.

The User Experience Barrier

Even well-intentioned sizing tools often fail due to poor user experience:

Complex onboarding: Requiring 15+ measurements before providing any value creates abandonment
Mobile unfriendliness: Many tools don't function properly on smartphones, despite 70% of fashion e-commerce occurring on mobile devices
Inconsistent availability: Some tools only appear on certain product pages, leaving shoppers confused about when they can access sizing help
Accuracy questions: Without clear explanations of methodology, shoppers don't trust the recommendations

During user testing sessions I've conducted, the average consumer abandons sizing tools after 2-3 minutes if they don't immediately see value. Complexity is the enemy of adoption.

The Measurement Challenge: Getting Accurate Body Data

Even with perfect sizing algorithms, the fundamental challenge remains: obtaining accurate body measurements from consumers.

The Self-Measurement Problem

Asking consumers to measure themselves produces notoriously unreliable data. Research from the University of Manchester's School of Materials indicates that self-measurements can deviate by 3-7cm from professional measurements, with common errors including:

Mirror measurements: Taking measurements while looking in a mirror creates parallax distortion
Incorrect tape placement: Bust measurements taken over thick clothing, waist measurements at the natural waist instead of the trouser waist
Inconsistent tension: Pulling tape too tight or too loose
Breathing and posture: Measurements change significantly based on posture and breathing

In my fitting room experience, even fashion industry professionals struggle to measure themselves accurately. The logistics of holding a tape measure, maintaining proper tension, and reading the measurement simultaneously while ensuring the tape remains parallel to the floor is genuinely difficult.

3D Body Scanning: The Future That's Always Five Years Away

3D body scanning technology promises perfect measurements captured in seconds. Companies like Bodymetrics and Size Stream have developed sophisticated scanning systems that can capture over 150 body measurements with millimetre precision.

The catch? Accessibility. Professional 3D body scanners cost £15,000-50,000 and require dedicated space. While some shopping centres and flagship stores have installed scanners, the technology remains inaccessible to the vast majority of consumers shopping from home.

Smartphone-based scanning apps have emerged as alternatives, using camera technology to approximate body measurements. However, accuracy remains inconsistent, with variables like lighting, camera quality, clothing, and user technique all affecting results. After testing multiple scanning apps with professional measurement validation, I've found deviations of 2-5cm are common—enough to recommend the wrong size.

The Reference Size Method: A Practical Alternative

The most practical approach I've encountered combines two data sources: basic measurements (which consumers can take with reasonable accuracy) and reference sizes from brands they already know fit well.

For example:

A shopper knows they wear a size 10 in H&M jeans and a size 12 in Topshop jeans
They can measure their waist with reasonable accuracy (within 2cm)
An intelligent system combines both data points to recommend sizes across other brands

This hybrid approach acknowledges human measurement imprecision while leveraging experiential knowledge. It's not perfect, but it's practical and accessible.

The Environmental and Economic Imperative

The sizing problem extends far beyond consumer frustration. It represents a significant environmental and economic crisis.

The Returns Crisis

According to data from the British Retail Consortium:

30-40% of online fashion purchases are returned
Each returned item generates approximately 20kg of CO2 from transportation
Returned items often cannot be resold at full price, creating waste
Processing returns costs retailers an average of £10-15 per item

Multiply this across the UK's £60 billion online fashion market, and the scale becomes clear. Sizing-related returns alone cost the industry billions while generating unnecessary carbon emissions equivalent to a medium-sized city's annual output.

The Overproduction Problem

Sizing uncertainty drives a secondary issue: wardrobes full of unworn clothing. Research from WRAP (Waste and Resources Action Programme) indicates that the average UK consumer has £285 worth of unworn clothing, with poor fit cited as the primary reason for non-wear.

When shoppers feel uncertain about size, they often purchase multiple sizes with the intention of returning the others—a practice called "bracketing." However, many consumers never complete the return, leaving clothes unworn in wardrobes. This drives overproduction as retailers compensate for perceived demand that doesn't reflect actual consumption.

From my work with sustainability initiatives in fashion, addressing fit accuracy represents one of the highest-impact interventions for reducing the industry's environmental footprint.

Tellar.co.uk: A Consumer-First Solution

Against this backdrop of sizing chaos and inadequate solutions, a genuinely different approach has emerged. Unlike the fragmented B2B tools and clunky interfaces that dominate the market, Tellar.co.uk has built something fundamentally consumer-focused.

The Key Differentiators

Comprehensive Brand CoverageWith over 1,500 brand-specific size charts integrated into the platform, Tellar.co.uk covers the vast majority of where UK consumers actually shop. This isn't a selection of premium retailers who've paid for integration—it's comprehensive coverage built for the consumer, not the retailer.

From my analysis of the platform, this breadth of coverage solves the fragmentation problem that plagues competitor tools. A single profile works across Zara, ASOS, Next, H&M, Boden, and hundreds of other brands. The consumer creates one sizing profile and receives recommendations everywhere they shop.

Measurement-Based IntelligenceRather than relying solely on self-reported measurements (with their inherent inaccuracy), Tellar.co.uk employs a hybrid approach. Users can input measurements, but they can also indicate sizes that fit well in brands they already know. The platform's algorithm cross-references these data points against its comprehensive database to recommend sizes with higher accuracy than measurement-only approaches.

This methodology aligns with best practices I've observed in professional fit analysis: multiple data points produce more reliable outcomes than any single measurement source.

True Consumer AccessibilityPerhaps most significantly, Tellar.co.uk is completely free for consumers. This isn't a freemium model with limited functionality—it's genuinely free, full-featured access. This matters because sizing assistance shouldn't be a premium service. Every shopper deserves to know what size to buy, regardless of whether they're purchasing from a retailer who's invested in enterprise sizing technology.

The platform works across both web and mobile, recognising that modern shopping is omnichannel. The upcoming Chrome extension takes this further, automatically identifying the correct size as shoppers browse product pages—effectively bringing sizing intelligence directly into the shopping journey without requiring users to switch between tabs or apps.

UK Sizing ExpertiseTellar.co.uk is built specifically for the UK market, understanding British sizing conventions and the brands that dominate UK wardrobes. While other platforms attempt global solutions that work poorly everywhere, Tellar's focused approach delivers better accuracy for UK shoppers.

Having worked with both UK and international sizing systems throughout my career, I can attest that regional focus matters significantly. The brands, sizing conventions, and body proportions that matter in the UK differ from those in the US, Germany, or Japan. A tool built for the UK consumer, understanding UK brands and sizing practices, delivers materially better results.

The Broader Impact

What excites me most about Tellar.co.uk from a technical perspective isn't just the consumer benefit—it's the potential industry transformation.

When consumers consistently purchase the correct size:

Retailers reduce return rates, improving profitability and sustainability
Consumers waste less time and money on poor-fitting purchases
Brands receive clearer demand signals, enabling better inventory planning
Environmental impact decreases as unnecessary production and logistics decline

Sizing technology shouldn't be a competitive advantage hoarded by retailers with large technology budgets. It should be a public good, accessible to all consumers regardless of where they shop. Tellar.co.uk's approach—building for the consumer first rather than the retailer—represents a fundamental shift in how we think about solving the sizing problem.

Practical Guidance: Getting Your Size Right Today

While technology continues evolving, consumers can take immediate steps to improve sizing accuracy.

Taking Accurate Measurements

If you choose to measure yourself, follow these professional techniques:

Essential measurements:

Bust: Measure around the fullest part of your bust, wearing a non-padded bra. Keep the tape parallel to the floor and neither tight nor loose—you should be able to slip two fingers under the tape.
Waist: Contrary to popular belief, the "waist" for clothing purposes is not your natural waist (the narrowest point). For trousers and skirts, measure where the garment will sit—typically 2-3cm below your navel.
Hips: Measure around the fullest part of your hips and bottom, typically 20-23cm below your waist measurement point.
Inside leg: Measure from your crotch to the floor while standing straight in bare feet. This is essential for trousers.

Professional tips:

Measure over thin, fitted clothing rather than naked (it's more accurate as clothing prevents the tape from sinking into soft tissue)
Take measurements at the same time of day, as bodies fluctuate throughout the day
Have someone else measure you if possible
Take three measurements of each dimension and use the median value
Record measurements in a note on your phone for easy reference

Building a Size Reference Database

Create your own personal sizing database:

Audit your current wardrobe
Note brands and sizes that fit well in different garment categories
Distinguish between tops, trousers, and dresses, as your size varies between categories
Include notes about style (e.g., "H&M skinny jeans size 10 fit perfectly; H&M straight-leg jeans need size 12")

This personal database becomes invaluable when shopping new brands. Tools like Tellar.co.uk can use this information to provide recommendations, but even without technology, understanding your size patterns helps predict what will fit.

Brand-Specific Considerations

Through years of fit analysis, I've observed consistent patterns across major UK brands:

Zara: Tends to run small, particularly in tops. Size up from your usual UK size.

H&M: Variable by collection. Basics run true to size; trend pieces often cut smaller.

ASOS: As a marketplace, sizing varies by brand. Always check whether you're buying ASOS own-brand (generally true to size) or a third-party brand.

Marks & Spencer: Tends to run generous compared to high street average. Often true to traditional UK sizing.

Next: Reasonably consistent and true to UK size standards.

Boden: Runs slightly generous, particularly in classic pieces. Size down for fitted styles.

These are generalisations based on my professional experience, but individual style and fabric choices always introduce variation.

Reading Between the Size Chart Lines

When consulting size charts, look beyond the numbers:

Check the measurement methodology: Some brands measure garments laid flat (then double for circumference), while others provide body measurements. This creates confusion when comparing charts.

Examine fabric composition: Anything over 3% elastane will stretch. You can size down slightly in stretch fabrics, but avoid sizing down in 100% cotton or linen.

Consider garment style: A "relaxed fit" size 10 might accommodate the same body as a "slim fit" size 12. Style descriptors matter as much as size numbers.

Look for actual dimensions: If a size chart provides the actual garment measurements (sleeve length, front rise, inseam), compare these to garments you already own and love. This is more reliable than body measurement recommendations.

The Future of Fashion Sizing

Looking ahead, several developments will reshape how we approach sizing.

AI and Machine Learning

Artificial intelligence is already improving sizing recommendations through:

Pattern recognition: Analysing millions of purchases and returns to identify which body types work best with which sizes and styles
Predictive algorithms: Learning individual preferences (e.g., someone who prefers looser fits vs. someone who prefers fitted)
Image analysis: Computer vision systems that can analyse garment images to predict fit characteristics

However, AI requires massive datasets to function effectively. Platforms with consumer-first data collection strategies, rather than retailer-siloed data, will deliver superior results as they can learn across the entire fashion landscape.

On-Demand Manufacturing

Made-to-measure technology is becoming more accessible. Companies like Unmade and Knyttan can produce custom-sized knitwear without the traditional cost premium. As manufacturing technology advances, we may see a future where standard sizing becomes obsolete, replaced by accessible, affordable custom sizing.

However, this transition will take decades. In the interim, effective sizing technology remains essential.

Regulatory Intervention

Consumer advocacy groups in both the EU and UK have called for sizing standardisation regulations. While previous voluntary standards failed, mandatory sizing transparency requirements could emerge, particularly as consumer protection and environmental concerns converge.

Potential regulations might include:

Mandatory publication of actual garment measurements
Standardised measurement methodologies
Clear labelling of fit type (slim, regular, relaxed)
Country of fit (acknowledging that sizing systems differ regionally)

However, regulatory change in fashion moves slowly. Technology solutions that work within the current chaotic landscape provide more immediate value.

Conclusion: Empowerment Through Information

After fifteen years in fashion technology and fit analysis, I've come to view sizing inconsistency not as a frustrating quirk of the industry but as a solvable information problem. The fashion industry has created chaos through vanity sizing, inconsistent manufacturing, and fragmented standards, but technology can restore order.

The key insight is this: sizing information exists. Every brand knows exactly how their garments measure. The problem has been making this information accessible, understandable, and actionable for consumers at the moment of purchase.

Solutions that treat sizing as a consumer problem requiring consumer-focused tools—like Tellar.co.uk—represent the path forward. Not fragmented B2B tools that work only where retailers choose to implement them. Not static size charts that require expert interpretation. Not scanning technology that's accessible to only a tiny fraction of shoppers.

What's needed is comprehensive, intelligent, accessible sizing information that works everywhere consumers shop, costs them nothing, and integrates seamlessly into their shopping journey.

For the first time in the digital fashion era, that future is becoming reality. Whether you're shopping for a special occasion dress, restocking basic t-shirts, or exploring a new brand, knowing your size shouldn't be a gamble. It should be a certainty.

The fashion industry's sizing chaos won't disappear overnight. Brands will continue pursuing vanity sizing strategies, manufacturing will always involve tolerances, and international expansion will keep creating regional fit differences. But with the right tools and information, consumers can navigate this chaos with confidence.

Your size isn't mysterious. It's just been unnecessarily complicated. With proper information and intelligent tools, getting your sizing right every time isn't aspirational—it's achievable today.

Sources and Further Reading

British Retail Consortium (2024), "Online Shopping and Returns Report"
WRAP (2023), "Valuing Our Clothes: The Cost of UK Fashion"
University of Manchester, School of Materials (2022), "Accuracy in Self-Measurement Studies"
London College of Fashion (2023), "The Evolution of UK Sizing Standards"
European Committee for Standardization (2007), "EN 13402: Size Designation of Clothes"
ASTM International, "ASTM D5585-95 Standard Tables of Body Measurements for Adult Female Misses Figure Type"
International Organization for Standardization, "ISO 8559: Garment Construction and Anthropometric Surveys - Body Dimensions"
McKinsey & Company (2024), "The State of Fashion: Returns and Sustainability"

About the Author

Ella Blake is a technical fashion stylist with fifteen years of experience across high street and luxury brands. She has worked extensively in pattern grading, fit technology implementation, and consumer sizing research. Ella holds certifications in garment construction and fit analysis from the London College of Fashion and has consulted for major UK retailers on sizing strategy and technology adoption. She specialises in bridging the gap between technical fashion knowledge and consumer accessibility.

This article was prepared fas a draft submission for The Times in October 2025 and reflects current industry practices and available technology solutions.