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Precious Metal Weights & Conversions

I've been researching this whole "troy ounce" versus "avoirdupois ounce" thing. I've always heard of troy ounces - and I know that is how precious metals like gold and silver are sold. Yet it's kind of like nautical miles - unless you have a need to know and are exposed to it's use, a unit of measure like troy ounces or nautical miles is just someone else's unit of measure. But how do troy ounces relate to "regular"[sic] avoirdupois ounces?

My desire to research all this came after reading several articles about the coming fall of civilization as we know it - the second Great Depression as some call it - and the collapse of the U.S. monetary system in general. One popular defense against losing your hard earned money and savings is to use your money to buy old silver coins.

The argument is made that the U.S. monetary system used to be, and still should be, based on a dual metal system using silver and gold - just as our founding fathers intended. After the looming collapse of our economy, it is likely that we will return to purchasing things with silver and gold - a reasonable assumption I guess.

Most larger denomination U.S. coins made prior to 1965 are 90% silver. These coins include the dime, quarter, half dollar, and silver dollar. A dollar of face value of these coins - whether it be ten dimes, four quarters, two halves, or just one silver dollar - contains 0.715 ounces of pure silver.[1] So, five dimes has the same silver content as two quarters or one half dollar - amazing how that works!

Some sites[2] [3] disagree slightly with the 0.715 conversion factor and state that the 0.715 is approximate - that when newly minted a face value dollar of these coins would be 0.723 ounces of pure silver. The difference in the factors being wear on the coins - and the different denominations have different amounts of wear based on common circulation - dimes and quarters circulated more than half dollars or silver dollars.

Measurement Systems

Avoirdupois is the system used in the U.S. and formerly in the UK. This is the system where

  • 1 ounce = 28.349523125 grams
  • 16 ounces = 1 pound
  • so doing the math, 1 pound = 453.5923699 grams
  • going the other way, 1 avoirdupois pound = 14.583333 troy ounces.

The Troy system dates from the Middle Ages, and is slightly different

  • 1 troy ounce = 31.1034768 grams
  • 12 troy ounces = 1 troy pound
  • again, doing the math, 1 troy pound = 373.2417216 grams
  • going the other way, 1 troy pound = 13.1657149 avoirdupois ounces

So, as you can see

  • 1 troy ounce is approximately 9.71%[4] larger than a avoirdupois ounce
  • however, 1 troy pound is approximately 17.71%[5] smaller than a avoirdupois pound.

Common Practices

Most sellers of these old 90% silver coins sell them in face value bags. A whole bag is $1,000 face value - using the above conversion factor, that is approximately 715 ounces of pure silver. Smaller bags are available from some sellers - half bags at $500 face value, quarter bags at $250 face value, and 1/10 bags at $100 face value.

Using the approximate conversion factor of 0.715, it's easy to calculate the number of troy ounces of silver in these standard bags. Some sellers however, especially on eBay, want to sell them by the pound - I've seen listings for these coins sold in both avoirdupois and troy pounds. This incongruity is what prompted me to research this topic - looking for a common conversion to pure troy ounces of silver.

1 avoirdupois pound of these 90% silver coins contains

  • 453.59 grams total weight
  • or 14.5833333 troy ounces
  • factor in the 90% silver and you get 13.125 troy ounces of silver

1 troy pound of 90% silver coins contains

  • 373.24 grams total weight
  • or as we stated above, 12 troy ounces
  • then factor in the 90% silver ratio and you get 10.80 troy ounces of silver

A Handy Reference Chart

Weight Convert to grams Convert to Troy oz. * 90% Pure Silver
1/4 Avoirdupois Pound 113.4 grams 3.65 Troy oz. 3.28 Troy oz. pure silver
1/2 Avoirdupois Pound225.8 grams7.29 Troy oz.6.56 Troy oz. pure silver
1 Avoirdupois Pound456.59 grams14.58 Troy oz.13.125 Troy oz. pure silver
With the Troy system, there is no need to convert to grams, done here for comparison purposes.
1/4 Troy Pound93.3 grams3.00 Troy oz.2.70 Troy oz. pure silver
1/2 Troy Pound186.6 grams6.00 Troy oz.5.40 Troy oz. pure silver
1 Troy Pound373.24 grams12.00 Troy oz.10.80 Troy oz. pure silver

And while I'm at it, though probably not necessary, let me add the following approximations based on the discussion above:

Face Value BagTroy Oz Pure Silver
$1,000715 ounces
$500357.5 ounces
$250178.75 ounces
$10071.5 ounces

Final Thoughts

While I'm thinking about it - just for the record

  • 1 nautical mile = 1852 meters
  • 1 statute mile = 1609.344 meters
  • doing the math, 1 nautical mile is 15.08%[6] longer than 1 statute mile.

[2] http://cmi-gold-silver.com/buy-silver-bullion-coins.html
[3] http://www.certifiedmint.com/silver.htm#Pre-1965%20U.S.%20silver%20coins-Circulated%20and%20Uncirculated
[4]more precisely 9.714286%
[5]more precisely, 17.714286%
[6]more precisely, 15.077945%

Other References