While this may seem easy to experienced gold buyers, finders, and sellers, figuring out the percentage of gold contained in any and all pieces of gold (jewelry and otherwise) is vital to figuring out the gold melt value of an item.
Luckily, it's really easy to do, even if you're dealing with vintage pieces of low karat purity or modern and/or repaired pieces that contain less gold than the stamped gold purity implies.
To find the gold content of any karat of gold jewelry, simply divide the karat purity by 24.
Gold Content = Karat / 24
24 karat gold is considered pure gold (which is why you may see 24k gold-plated jewelry, but will not see 24k SOLID gold jewelry... pure gold is too soft).
So in the world of gold purity markings, the karat purity is always based on a total of 24 and goes down in a sliding scale mostly on even numbers (but not always, especially with older pieces).
The alternate to knowing the gold content formula mentioned above, is memorizing the gold contents of each karat purity. Definitely possible, and many people do memorize the gold contents, but it's still valuable to know how to calculate gold content, especially if you're dealing with an odd karat purity or simply forget one of the numbers.
For reference, here are the common gold karat purities and their corresponding gold content....
10k - 41.7% pure gold
12k - 50.0% pure gold
14k - 58.3% pure gold
18k - 75.0% pure gold
22k - 91.67% pure gold
Another time this formula comes in handy is when you suspect a piece of jewelry may contain less gold than the stamped purity. This is often the case with repaired pieces of gold jewelry... the repair will almost always contain solder and may also contain gold of a lower purity.
In the U.S., it is legal for gold makers to "under-karat" their jewelry by up to 1/2 karat. This means that gold jewelry makers can use a 13.5 karat gold mixture and still call it 14 karat gold. Doesn't sound like much until you do the math and realize that's 2% less gold (which can add up substantially on large and heavy gold pieces).
If a piece of gold jewelry has been repaired, it can be up to 1 FULL KARAT LESS in gold purity and still retain the original marking.
This is all very valuable information to know before calculating the gold content and melt value, and especially before buying any piece of gold jewelry as refiners generally pay based on the amount of gold RECOVERED, and not based on the purity markings.
Hope this article helped add some knowledge to your gold scrapping toolkit.
Thanks for reading!