How do you figure the gold content of a 1918 Mexican 20 pesos?

Paul A. writes: I know from one of your previous answers that in order to find the approximate current value of a Mexican gold peso, I multiply the gold content (in that case, .121 pure gold troy ounce) against the world price of gold in dollars. In my case, I have a 1918 and a 1919 Veinte Pesos coin, each indicating “15 gr. oro puro.” What is the gold content of these coins, to be used for multiplying against the world price?

To get the bullion value of any gold coin use the following formula:

Total gross weight in grams x fineness/31.1033 (grams in a troy ounce of gold) x current price of gold in troy ounces.

Example: A U.S. $20 gold coin weighs 33.436 grams. The coin is .900 fine (90% gold). 33.436 x .900=30.0924 grams of pure gold. 30.092/31.1033= .9675 (rounded). We now know that the coin contains .9675 of a troy ounce of pure gold

.9675 x world gold price (let’s use $1200) = $1161. This is the intrinsic bullion value at $1200 gold.

The above calculation does not include numismatic value which is a premium above the gold content. Numismatic value depends on market forces, market demand for a specific date and mint mark and the grade (condition) of the coin. For example typical uncirculated, common date St. Gaudens $20 gold coins are currently trading between $1300 – $1320, about $100 over the gold content. High quality uncirculated pieces such as ones that grade MS65 bring about 30-40% more.

To use your Mexico example: Your coin already states the net gold weight, “15 gr. oro puro.” (15 grams of pure gold). To get the actual bullion value: 15/31.1033 = .4823 .4823 x $1200 (current price of gold)= $578.76. Again this does not include any numismatic market premium which may or may not exist for that coin.