Last month, I was browsing the LK Bennett sales, and picked up an indigo pleated skirt in what I vaguely hoped would be the right size. But as I headed for the changing room, I had a feeling it would be too small. And as I struggled with the zip, I raged for the millionth time that a size 10 should be a size 10.

As everyone who's ever bought clothes knows, high-street sizing is completely mad. You can easily be a size 8 in one store, and a size 14 in another, and it's impossible to guess your size without lots of zip-wrangling. However, I am a computer programmer as well as a fashion fan, and so I decided to do something about it.

I collected the official size data published by many different stores - LK Bennett, like many others, publishes its sizes online. Then I built a data visualisation to help the women of Britain and the US find their best sizes at different shops. It's called What Size Am I?

Fashion Sizes Chart. Click the image or here to view the full interactive chart and find out where to get the best fitting clothes for you.

My app lets you put in your measurements (bust, waist and hips) in inches and cm and see your most perfect fit in tops, skirts and dresses at shops from ASOS to Zara. If you're using a modern web browser, you'll see a nice interactive graph of where you fit in.

I tested the app with friends, and while some felt that fit also depends on fabric and cut, we were all interested to see the variation between shops - "I knew M&S sizes were large!" said one. I also adapted it to display nicely on mobile phones, so you can use it on the move.

The data I collected confirms that sizing is indeed madness. In the UK, a size 16 at Jaeger has a bust of 108cm, waist of 88cm and hips of 114cm: a size 16 at Banana Republic has a bust of 98cm, waist of 77cm and hips of 103cm. That's a 4-inch difference, and it's not unusual.

I assumed that the pricier stores would size smaller, but that's not actually true. Counter-intuitively, a size 10 in upmarket Whistles, Zara, or Reiss is actually quite a bit bigger than a size 10 in ASOS, Monsoon, or M&S. And mass-market Next consistently has the smallest sizes on the high street.

I was particularly interested to see the different body shapes flattered by different stores. Broadly, M&S, Karen Millen and French Connection look the most pear-shaped: Banana Republic and Warehouse are best for the top-heavy: LK Bennett and Zara are cut for a fitted waist, while Oasis and TopShop seem most up-and-down.

I'm surprised no-one has explored the data or built a similar site before, but then being a computer programmer and a fashion fan is… unusual. Coders get an unfair press in some ways (I find most are articulate and charming) but when it comes to fashion, the clichés are largely true. Tech conferences are a sad sea of baggy black T-shirts.

And therefore, programmers do not often write interesting fashion apps, and the fashion world ignores those of the dorky persuasion. This is a shame, because I see potential fashion hacks everywhere and hear about few. And too many fashion sites are bloated, Flash-heavy nightmares that hinder rather than help users.

But really, fashion and programming should have a natural affinity: at their best, both are about craftsmanship, invention and delight in the new. So, fashion firms, don't turn up your noses at the nerdy, despite their black T-shirts: seek out the geeks.

Take a look at the data I collected from highstreet stores and tell me what you think in the comments below.

This text is true as of the publication date, however it may change as stores change their sizings. The app and spreadsheet will be more up-to-date.

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