Where the number on the label comes from
Hardly anyone knows this and it is simple: in menswear, the European size is half the chest measurement in centimetres. A 50 is a 100 cm chest, a 54 a 108 cm one. In womenswear six is subtracted from that half, because the scale was fitted to a different block: an 88 cm chest is a 38.
The other scales are that one shifted. The British women's scale runs twenty-eight numbers below the European one and the American thirty; in menswear both label directly by chest inches, which is the same measurement in another unit. Italy runs four numbers above Europe, which is why an Italian 42 is a Spanish 38.
How to take the measurement
Chest at its widest, with the tape level all the way round and snug but not tight, over a thin layer rather than over a jumper. Breathe normally: holding your breath in gains a couple of centimetres that no garment will give back. For trousers what governs is the waist, and for dresses, hips as well.
One measurement is not a whole size. Two people with the same chest and different shoulders wear different shirts, which is why the number is a starting point and not a verdict.
The formulas, to do it by hand
- Menswear: half the chest. 100 cm → size 50.
- Womenswear: half the chest minus six. 88 cm → 44 − 6 = size 38.
- Italy: European + 4. A 38 is an Italian 42.
- Women's UK: European − 28. Women's US: European − 30.
What is worth knowing
No standard obliges anyone to any of this. EN 13402 exists, gives sizes in body centimetres and is barely used, so the scales above describe what shops do rather than what a norm requires. Add vanity sizing on top — labelling a garment smaller than it measures, so the customer feels better — and whole scales have drifted over the decades: a US women's 8 from 1960 measures roughly what a 4 measures today.
So take the number as an orientation and the measurement as the fact. Where a brand publishes its own size guide in centimetres, that guide beats any general table, this one included.