Men’s dress shirt collar sizes in the US and UK are measured in inches; the EU uses centimeters. A 15.5" collar is an EU 39 or 40 depending on the brand’s rounding.
Collar size is one of the few clothing measurements in this chart that's genuinely close to a formula: the EU centimeter figure is just the US/UK inch measurement multiplied by 2.54 and rounded to a whole or half centimeter — a 15" collar is 38.1 cm, a 16" collar is 40.6 cm.
Rounding is where brands disagree slightly: a 15.5" collar (39.4 cm) gets rounded to EU 39 by some brands and EU 40 by others, since it falls almost exactly between the two whole-centimeter sizes. If you're between two EU sizes, sizing up is the safer choice for a collar, since a tight collar is far less comfortable than a slightly loose one.
| US | EU |
|---|---|
| 14 | 36 |
| 14.5 | 37 |
| 15 | 38 |
| 15.5 | 39/40 / 39 / 40 |
| 16 | 41 |
| 16.5 | 42 |
| 17 | 43 |
| 17.5 | 44/45 / 44 / 45 |
| 18 | 46 |
Clothing sizes are brand-assigned labels, not fixed measurements — use this as a starting point, not a guarantee.
A 15.5" collar is 39.4 cm, which different brands round to either EU 39 or EU 40 — check the specific brand if you’re between the two.
Close to one — EU cm is the US/UK inch measurement times 2.54 — but brands round differently near the half sizes, so this chart still lists it as a lookup rather than telling you to calculate it yourself.
Measure around the base of your neck where a buttoned collar would sit, then compare to the nearest size on the chart — err on the larger side if you’re between two.