Three different number systems are used to express how pure a piece of gold is:

  • Karat (or carat in UK English) — a 24-point scale. 24k = pure gold.
  • Millesimal fineness — parts per thousand. 999.9 = pure gold.
  • Percentage — straight per cent purity. 99.99% = pure gold.

All three say the same thing in different units.

The conversion table

KaratMillesimalPercentageCommon use
24k999.9 (or 999)99.99%Fine gold — Britannia, Maple Leaf, PAMP minted bar
22k91691.67%Sovereign, Krugerrand, American Eagle
21k87587.5%Middle-East jewellery
18k75075%High-quality jewellery
14k58558.5%US jewellery standard
10k41741.7%US lower-end jewellery
9k37537.5%UK lower-end jewellery

How to calculate the value

The maths is the same in all three systems — only the multiplier changes.

Value = weight × (purity / 1000) × spot price per gram

Examples (using £85/g gold for illustration; check the live dashboard for current spot):

  • A 7g 18ct gold ring → 7 × 0.750 × £85 = £446.25 spot value.
  • An 11g 9ct gold chain → 11 × 0.375 × £85 = £350.62 spot value.
  • A 31.10g (1oz) 24ct gold bar → 31.10 × 0.999 × £85 = £2,641 spot value.

The bullion value calculator does this automatically — pick the karat from the dropdown, type your weight, see the live spot.

Why “24” karat for pure gold?

The system dates to medieval Europe. A carat (from the carob seed historically used as a weight standard) became the standard fraction for grading gold purity. 24 was chosen so the fractions worked cleanly — 12k is half pure, 6k is a quarter, 18k is three-quarters.

The modern millesimal system replaced karats in industry because it’s simpler arithmetically. UK hallmarks have used fineness numbers instead of karat numbers since 1999, but jewellery is still routinely described in carats colloquially.

See UK hallmark guide →

Karat for silver?

The karat system is for gold only. Silver uses fineness numbers directly:

  • Fine silver — 999.
  • Britannia silver — 958.4.
  • Sterling silver — 925.
  • Coin silver — 900.
  • Continental silver — 800.

Why bullion coins use 22k (916) not 24k (999.9)

22ct gold is harder than pure 24ct gold. The Sovereign and Krugerrand use 22ct so they wear well in circulation — pure 24ct gold is soft enough to be marked with a fingernail. To keep their stated fine gold content (e.g. exactly 7.32g for a Sovereign), the coins are physically a bit heavier than their fine content: a Sovereign weighs 7.99g total to deliver 7.32g of pure gold.

Modern non-circulating bullion coins (Britannia 24ct, Maple Leaf 24ct, Buffalo 24ct) skip the alloying since they’re not designed to be carried in change.

Rule-of-thumb resale ratios

UK scrap dealers tend to pay:

  • 80–95% of spot for 18ct and 22ct, depending on quantity and quote source.
  • 70–85% of spot for 9ct (lower because it has to be refined more heavily).
  • 60–80% of spot for 14ct mixed jewellery (less common in the UK).

Compare offers and never accept the first quote — there’s typically 5–10 percentage points of spread between competing dealers.