Why does your clothing size change from brand to brand?

By Liza Colbeck, founder of IN OUR NAME

If your wardrobe holds three different sizes, you are in good company. You find something you love, choose the size that usually fits, and discover that this particular brand has other ideas.

A size 12 at Topshop and a size 12 in Marks & Spencer's own collection need not fit the same way. In the US, someone shopping at Eileen Fisher and Anthropologie may find themselves making a similar adjustment.

The reason goes deeper than the number on the label. Brands build their sizing around different customers, body proportions and ideas of fit. Then we, as shoppers, are left to work out where we belong in each of their charts.

I spent part of my career looking closely at what sits behind those differences.

SizeUK: understanding who we were actually dressing

I chaired the Data Analysis Committee for SizeUK, the UK Government-supported programme that scanned more than 11,000 people using 3D body scanning technology.

Working with major retailers, my role was to lead the interpretation of those scans: turning an enormous amount of information into decisions about how clothes should fit, how sizes should change and what each shop should stock.

We could look beyond a handful of measurements and understand much more about shape. Two people might have the same waist measurement, but very different bust proportions, hip shapes or torso lengths. That matters when you are making something they both need to feel comfortable wearing.

The research was British, but the principles apply wherever you are in the world. Good fit starts with understanding the people you are dressing.

What did the data actually change?

Stock that reflected the people shopping locally

Body size and shape varied by region. In the analysis, Scottish male shoppers tended towards larger, taller proportions, while Leicester's population tended towards a slighter build.

That gave retailers a practical reason to send a different mix of sizes to different stores, rather than dispatching the same proportions of small, medium and large nationwide.

For the customer, the benefit was straightforward: a better chance of finding the right size in their local shop. For the retailer, it meant stock decisions could reflect local demand more closely.

Lingerie developed around more than a cup size

For retailers developing lingerie, the scans offered valuable detail about bust point position in relation to the rest of the body, and how support requirements changed as cup size increased.

A cup size alone could not tell the whole story. Understanding where the bust sat, and its relationship to the wearer's other dimensions, helped inform how a bra needed to be constructed.

Anyone who has spent the day adjusting an uncomfortable bra will understand why that matters. Getting those details right also makes commercial sense: fit gives customers a reason to come back.

More thoughtful changes between sizes

"Grading" is the term for how a garment's dimensions change from one size to the next.

It is more involved than making everything a little wider. Bodies do not change in equal increments at the bust, waist and hip, and proportions can differ across age groups.

This is where the Topshop and Marks & Spencer example becomes useful. Brands with different customer profiles may need different starting proportions and different grading rules. A size 12 can therefore represent two quite different garments, even within the same country.

The same principle applies when shopping across US brands such as Eileen Fisher and Anthropologie. Knowing your size in one does not automatically tell you which size will work in the other. The customer profile, the silhouette and the intended fit all play a part.

Fit mannequins based on actual customers

Several retailers used the scan data to develop fit mannequins that reflected their customer profiles.

Alongside fittings on people, these provided a consistent reference that development teams could return to throughout the process, without having to arrange a live fitting for every check.

The useful shift was that those mannequins could represent the body shapes of the retailer's customers more closely. The information was helping shape the clothes themselves.

Why sizing still varies, wherever you shop

SizeUK gave retailers a better understanding of their customers. It did not make every retailer's customers, or their size charts, identical.

That distinction still matters. A shared number on a label does not mean two brands have used the same measurements underneath it.

Shopping internationally adds another layer. UK, US and European size conversions help you navigate the labels, but they cannot account for every brand's proportions or the way a particular garment is cut.

The lesson travels well beyond Britain: understand the customer, develop the fit accordingly, and give the person buying the clothes enough information to choose confidently.

What this means at IN OUR NAME

I bring the same attention to IN OUR NAME that I brought to SizeUK. Measurements matter, but so does understanding how they translate into something you will enjoy wearing.

Every product on our website has its own detailed size guide, with real measurements for that specific garment. A shirt dress and a fitted skirt ask different things of their fit, so one general chart can only tell you so much.

Whether you are shopping with us from the UK, the US or elsewhere, you can compare those measurements with a similar piece you already love wearing.

Colour and print may be what catch your eye. We want the fit to be part of what makes you reach for it again.

Read more about Liza's background: Meet Liza