Why does my 10-ounce silver bar weigh more than 10 ounces?
September 4th, 2008 | by mbhunter |
As I was surfing around some other money blogs I ran across an investor who just bought some silver. He weighed a 10-ounce silver bar and found that "strangely" it weighed 10.9 ounces.
His scale is fairly accurate — probably better than a tenth ounce. What he actually measured was the weight of the silver bar in avoirdupois ounces — what Americans would call "ounces" — instead of troy ounces. The conversion is
10.9714286 avoirdupois ounces = 10 troy ounces.
The avoirdupois system, as one might suspect from the name, originated in France. The word comes from avoir de pois, meaning "goods by weight." It was adopted by the British and later by the United States. The Troy system is also French, named after the commune of Troyes, France. This system of measure is used for black powder, gemstones, and precious metals.
The connection between the two systems is the grain. There are 7,000 grains in an avoirdupois pound, which is 16 avoirdupois ounces (oz). There are 5,760 grains in a troy pound, which is 12 troy ounces (ozt). So
1 ozt = 5,760 x 16 / (7,000 x 12) oz = 1.09714286 oz.
In terms of grams, 1 ozt = 31.1034768 grams.
This distinction is important, because silver (as well as gold, platinum, and palladium) prices are quoted per troy ounce. It’s possible that someone might pay too much for silver if regular ounces were used instead of troy ounces. Even worse, if an item was an alloy of silver but was treated as pure silver, the person would pay that much more. (Pre-1965 US dimes, quarters, and half-dollars are 90% silver. Sterling silver is 92.5% silver.)
This investor looks like he’s enjoying his silver, and this is why it looked like he got an extra couple of ounces of silver for his money. Wouldn’t that have been nice? 
6 Responses to “Why does my 10-ounce silver bar weigh more than 10 ounces?”
By Matthew Sumpter on Sep 4, 2008 | Reply
Thanks for the information. I didn’t know this, but it is very interesting.
By mbhunter on Sep 4, 2008 | Reply
Thanks for stopping by Matthew!
I don’t have any 10-ozt bars. I did see a 1,000-ozt bar on eBay once … offered to split the cost with another friend, but we’d need a band saw.
By Early Retirement Extreme on Sep 4, 2008 | Reply
You usually pay for the silver content only, if it’s not pure. If it’s already marked in troy ounces (like the spot prices are), there’s no confusion. If it’s not (like junk silver), it’s usually never sold in troys, but in pounds or kilos of silver content. Actually it’s the same for gold. A Krugerrand is not a pure as a Maple so the former would be slightly heavier (and sell at a discount because it’s not pure).
By Todd A on Sep 4, 2008 | Reply
Thank you for the interesting post. I had no idea an ounce wasn’t an ounce in all cases !
By terry on Sep 21, 2008 | Reply
Why does my 10-ounce silver bar weigh more than 10 ounces?
Because you are comparing apples to oranges.
By DJP on Feb 1, 2009 | Reply
Because bars aren’t sold in regular ounces like you’re thinking of. Precious metals are sold in Troy ounces (should be stamped on your bar).
1 troy ounce = 1.1 ounces