The three biggest sizing systems use different numbering logic. Here is how they line up and where they drift apart.
Three systems, three starting points
US womenswear numbering starts near zero and steps in twos. UK sizing runs four numbers higher for the same garment, while EU sizing is loosely derived from bust measurement in centimetres minus a constant. Because each system was standardised separately, a label alone never tells you the underlying measurement.
The usual offsets
For women's apparel the common rule of thumb is UK = US + 4, and EU = US + 30. Italian sizing adds another four numbers on top of EU, and French sizing usually matches EU. Menswear behaves differently: tops are largely driven by chest measurement and bottoms by waist measurement, which makes men's conversions more stable across borders.
Why the offsets break
Vanity sizing has slowly enlarged garments carrying the same label, especially in US retail. Two garments labelled US 8 from different decades can differ by several centimetres at the waist. Cut also matters: a relaxed shirt and a slim shirt with identical labels fit differently even from one brand.
A practical workflow
Measure your bust, waist and hips once and write them down. Convert the label you already own with a converter to get your starting point, then check the specific brand's own measurement chart before buying. If you sit between two rows, choose based on the fabric: rigid weaves favour the larger size, stretch fabrics the smaller.
Frequently Asked Questions
Is UK size always four numbers above US?
For womenswear it usually is, but the offset is a convention rather than a standard. Always confirm against the retailer's measurement chart.
Do men's sizes convert more reliably?
Generally yes, because men's shirts and trousers are labelled with actual measurements such as chest or waist rather than abstract numbers.