The gold measurements

All gold is not created equal. Identical rings offered in both 12-karat and 18-karat gold. . . the 18-karat ring has fifty-percent more of the precious metal than the 12-karat ring. A 10-karat piece of jewelry has less than fifty-percent gold in the metal that is used.

The measurement of the fineness or percentage of gold in a metal is a karat–abbreviated K. Pure gold–100 percent gold–is 24 karats. All alloys (the combination of gold and other metals) are calculated based on the 24 karats. So, if you are buying an 18K ring, you are purchasing 18 parts gold and 6 parts other metals.

Gold less than 10-karat (41.67% gold) cannot be legally called “gold” in the United States.

Karat Gold Fineness

Karat Percentage Gold
Fineness
Karat Percentage Gold
Fineness
8 33.33%
.3333
16 66.67%
.6667
9 37.50%
.3750
17 70.83%
.7083
Note: Gold Fineness less
than 10 karat cannot have
a karat mark within U.S.
18 75.00%
.7500
10 41.67%
.4167
19 79.17%
.7917
11 45.83%
.4583
20 83.33%
.8333
12 50.00%
.5000
21 87.50%
.8750
13 54.17%
.5417
22 91.67%
.9167
14 58.33%
.5833
23 95.83%
.9583
15 62.50%
.6250
24 100.00%
1.000