Karats → Millesimal → %

The karat (ct) is the historical unit for gold purity. 24-karat gold is pure gold (999‰), while 18-karat gold contains 75% fine metal (750‰). In Switzerland, the most common legal fineness grades are 750‰ (18 ct), 585‰ (14 ct), 375‰ (9 ct), and 333‰ (8 ct). Millesimal fineness (in parts per thousand) is the official standard used by the Federal Precious Metals Control Office.

The relationship between karats and millesimal fineness is linear: Fineness = (Karats / 24) × 1000. Conversely: Karats = (Fineness / 1000) × 24. The percentage is simply the fineness divided by 10. These conversions are essential for goldsmiths, jewellers, and dealers who work daily with alloys of varying purities.

In Switzerland, the official hallmark (St. Bernard's head from 1883 to 1995, then the cat's head since 1995) guarantees the fineness of the object. Six fineness grades are officially recognized for gold: 333, 375, 585, 750, 916, and 999. Any jewellery sold as 'gold' in Swiss commerce must have a minimum fineness of 333‰ (8 ct). Sworn cantonal assayers verify compliance using touchstone tests or XRF spectrometry.