BUYING clothes can be a maddening experience—even for a woman, and especially online. As many ladies will aver, different shops and fashion labels appear to have varying definitions of what a "size 6", say, is. Sure, clues sometimes lurk at the seams—or stashed away in the closet-equivalent of an online retailer's website. But they are not terribly user-friendly, nor is it easy to compare sizes across labels.

Fashion aficionados will therefore be thrilled to hear that Anna Powell-Smith, a London-based web developer, has just rolled out What Size Am I? Using the online app is a doddle. Simply adjust a set of sliders at the top of the screen to reflect bust, waist and hips (in metric or imperial units) and the software does the rest. It sorts through a selection of British and American labels to find the sizes in each which best match the buyer's vital statistics.

Ms Powell-Smith's data make it plain that the size disparity really does exist. In one store, for instance, a size 16 was fully 10cm larger than a 16 at another. Intriguingly, she also found that lower-cost, mass-market labels, which she had expected to have a more generous sizing policy, in fact carry smaller sizes than high-end retailers, at least in Britain.

The app is part of a growing trend to create interactive data-visualisation, or dataviz, projects that allow users to interact with large data sets in novel ways. Media organisations often use such tools to get a better handle on everything from government budgets to public-transit schedules. This can be laborious. Ms Powell-Smith trawled each store's website and compiled it into a spreadsheet.

It isn't just exasperated customers who will appreciate her effort. Mismatched sizes are a headache for online retailers, too. They complain about the huge volume of returned items: as high as 40% of all clothes sold over the internet, according to one German study.

Of course, some items are returned for reasons unrelated to size. Once the garment arrives a buyer may simply decide that, on reflection, it is not quite as becoming as had been hoped. A few European companies have ideas to prevent such surprises. One German start-up, UPCload, which officially launched this month, uses the customer's webcam to scan her (or his) body and creates an avatar which can then be dolled up and scrutinised. A similar firm in Estonia, called Fits.me, recently attracted over €1m ($1.3m) in venture capital. It offers online retailers a robotic torso that can be used to create virtual copies of the clothes they hold in stock.

For now Ms Powell-Smith's website caters only to the fairer sex. But she has got a number of e-mails asking her to include sizes for men (like Babbage) and children. The mismatches there, she noted, can be even more egregious than for ladies.