The short answer
1/20 12K GF means the piece is gold filled (GF), with a layer of 12 karat gold (12K) that makes up one twentieth (1/20) of the metal’s weight. The rest is a base-metal core, usually brass. Put together, about 2.5% of the metal is pure gold.
Breaking the mark down
1/20 — how much of the weight is gold layer
The fraction is the share of the item’s metal weight that is the karat gold layer. 1/20 is 5%. A thicker layer shows a bigger fraction, such as 1/10 (10%); a thinner one a smaller fraction, such as 1/40 (2.5%), which you’ll usually see on rolled gold plate rather than gold filled. Under the US trade rules most makers followed, an item had to have at least a 1/20 layer to be called gold filled.
12K — the purity of that layer
Karat measures purity out of 24. 12K is 12/24, or 50% pure gold. 10K is about 41.7% and 14K about 58.3%. The karat describes the layer only — not the whole piece.
GF — the construction
GF (or G.F., or “Gold Filled”) means the gold is a solid layer mechanically bonded to the core with heat and pressure — not a thin plated film. That’s what makes the gold recoverable. Compare RGP (rolled gold plate, usually a thinner bonded layer) and GP (gold plated, usually too thin to recover). See Gold Filled vs Gold Plated.
Working out the pure gold
Multiply the fraction by the purity: 1/20 × 12/24 = 1/40, or 2.5%. So:
- A 10 g 1/20 12K GF chain holds about 0.25 g of pure gold.
- A 100 g bag of 1/20 12K GF pieces holds about 2.5 g.
- A 1 kg lot holds about 25 g.
Weigh only the metal: glass, stones, springs and steel pins contain no gold. Our gold filled calculator does the maths for any mark and weight.
Other common marks, compared
| Mark | Layer | Purity | Pure gold in the metal |
|---|---|---|---|
| 1/20 10K GF | 5% | 41.7% | about 2.1% |
| 1/20 12K GF | 5% | 50% | about 2.5% |
| 1/20 14K GF | 5% | 58.3% | about 2.9% |
| 1/10 10K GF | 10% | 41.7% | about 4.2% |
| 1/10 12K GF | 10% | 50% | about 5% |
| 1/10 14K GF | 10% | 58.3% | about 5.8% |
| 1/40 12K RGP | 2.5% | 50% | about 1.25% |
| 14K GF (no fraction) | usually 1/20 | 58.3% | about 2.9% |
When a mark shows a karat but no fraction — such as 14K GF or 12K GF — the layer is normally taken to be the 1/20 minimum. Thickness is confirmed on assessment.
Variations you might see
- 1/20 12KT GF, 1/20 12K G.F., 12K GF — all the same idea, written differently.
- 1/20 12K GF on Sterling — a gold filled layer over a silver core rather than brass. Mention this when you sell, as the silver may also count.
- “Warranted 20 Years” on a watch case — usually a gold filled case. The years referred to how long the layer was expected to last.
- 12K GF next to a maker’s name such as Speidel, Krementz, Winard or Van Dell — common American gold filled makers.
Does 1/20 12K GF mean it’s worth selling?
Yes — it’s one of the marks we buy most. On its own a single chain holds only a fraction of a gram of gold, so gold filled is best sold in quantity, where the refining costs are spread across more gold. Read Is Gold Filled Jewellery Worth Anything? for why offers are based on recoverable gold rather than the spot price, or check your mark in the Will we buy it? checker.