A 12 in one shop is a 14 in another and a 10 in a third. This is not incompetence. There is simply no enforced standard, and there never has been.
What a size label actually is
In the UK there is a voluntary sizing guidance document, and nothing requiring a retailer to follow it. Each brand drafts to its own fit model — one real person whose measurements define the base size, which is then graded up and down by rules the brand chooses.
Two variables, both set commercially:
Which body the base size is drafted on. Different brands aim at different customers, so the same nominal size starts from different measurements.
How the grading works. Going from a 12 to a 14 might add two inches at the bust and one at the waist in one brand, and something quite different in another. This is why a size can fit you in one garment and not in another from the same shop.
Vanity sizing is real and measurable
Sizes have drifted larger over decades. A garment labelled 12 today is substantially larger than one labelled 12 in the 1970s.
It is a commercial decision — a customer who fits a smaller number feels better and buys more — and it is why the number tells you about the brand's positioning as much as about the garment.
The practical consequence is that comparing sizes across brands, or across decades, is meaningless. Comparing measurements is not.
What to use instead
Measure yourself once. Bust or chest, natural waist, hips, and inside leg. Write them down. How to measure yourself has the method.
Measure a garment that already fits. More useful still. Lay it flat and measure across the chest, the length from shoulder seam to hem, and the sleeve. Those three numbers are what you compare against a product page.
Read the garment measurements, not the size chart. A size chart describes bodies; garment measurements describe the thing you are buying, including the ease built into it. When a retailer gives both, the garment measurements are the useful ones.
Treat the model information as data. Height and size worn tells you how the garment is being shown. A dress on someone eight inches taller than you will not sit where the photograph suggests.
Why it is worse online
Two reasons. You cannot try it, and the photographs are taken on one body in one size — usually pinned at the back to fit perfectly, which is standard practice and worth knowing.
Reviews mentioning fit are the most valuable thing on a product page. Several independent people saying a garment runs small is better evidence than any chart.
Where it bites hardest
Shoes, where there is no standard at all and the last matters more than the number — buying shoes online covers it.
Trousers, because two measurements have to be right at once.
Anything structured — tailoring, coats, shirts — where the shoulder decides everything and cannot be altered easily.
Anything from a different market. UK, EU, US and Japanese sizing all differ, and conversion charts are approximations, not translations.
The habit worth building
Keep three numbers in your phone: your chest measurement, and the flat chest and length measurements of the garment you own that fits best.
That single note turns most online sizing decisions from a guess into a comparison, and it is the difference between a wardrobe and a pile of returns. Fit words translated covers the language on the rest of the page.