Troy ounces and Avoirdupois ounces

The one ounce bars above probably aren’t as heavy as you think they are. Image courtesy of Adobe Stock.

When I say I’m holding something that weighs one pound, how heavy do you think it is?

Well, you can sort of get a picture in your mind based on other things in your life that weigh one pound.

What if I mention I was holding a pound of silver? How much does that weight?

Ah, that is more complicated.

By long-standing tradition, precious metals are measured in troy weight system. Everything else is measured in the avoirdupois system.

I’ll go into some depth to explain my approach for this series of posts.

Troy system:

Precious metals are measured in the troy system.

There are 12 ounces in a pound and 28.35 grams in a troy ounce.

That would be 373.24 grams in a troy pound of precious metals.

Avoirdupois system;

Other things are measured in the avoirdupois system. There are 16 ounces in a pound (Aha! I hear you say. I knew 16 ounces per pound was in here somewhere!) To make this more complicated, weight of an avoirdupois ounce is 31.1 grams, also different from a troy ounce.

That makes 453.59 grams in an avoirdupois pound.

You can find a more detailed explanation at Wikipedia.

So we have:

measurement grams in oz oz in lb grams in lb
troy         31.10      12.00     373.24
avoirdupois         28.35      16.00     453.59
ratio     91.2%     133.3%     121.5%
norse         26.50
troy ounce in avoirdupois pound      14.58
Norse ounce in avoirdupois pound      17.12


Since most of us think in terms of the avoirdupois system, I’m currently thinking I might convert large weights in this series of posts from troy into avoirdupois.

For example, a gallon of water weighs 8.34 pounds. That would be 133.4 avoirdupois ounces or 3,782.9 grams.

If I said I was holding a bar of silver weighing the same as a gallon of water on my scale at home, 8.34 pounds, we could roll that 3,783.9 grams of silver into 121.7 troy ounces, or 10.1 troy pounds.

If I was holding that same bar of silver and you wanted to get a feel for the weight in terms of how you usually think, I could say that weighs 10.1 pounds (in troy system) and feels like the equivalent of 6.86 pounds (in the avoirdupois system).

So extending that idea out, if a Viking raiding party made off with 1,000 pounds of silver as a payoff for not looting a town, the Viking ship would have to carry another 686 pounds of weight on the way home.

To extend this illustration, if there were some event that happened which involved 1,000 coins at one ounce each in Norse weight, that would be 71 troy pounds, but if you put it on your scale at home it would register 58 pounds.

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