You would think it would be an easy question but I haven't been able to find a definitive answer. Coin sizes are almost always given in millimeters, e.g., a Cent has a diameter of 19mm. However, when the size was originally codified it was 0.75". It actually still is 0.75", with the 19mm being a close approximation (actually 19.05mm). Likewise, the Nickel, designed with a diameter of 0.835" is now designated with a diameter of 21.21mm. So, when did we (the mint) officially go from inches to millimeters as size designation?
Almost 30 years ago, in 1988, the Congress declared to use SI system (kg-meter), particularly in state departments, so, the mint too included. Dual (inch-lb and kg-mm) system has been a problem for the US in the World where the big majority are using SI now.
You know what they call a quarter pounder with cheese in France? … They don't call it a Quarter Pounder with Cheese? No, they got the metric system there, they wouldn't know what the [heck] a Quarter Pounder is. What'd they call it? They call it a ... Royale with Cheese.… - Pulp Fiction
It is the same with weights. They are always listed a ounces in the SCWC but most European coins are actually designed with a specific weight in grams. It's confusing.
The French call it: deux toutes les galettes de boeuf, sauce spéciale, laitue, fromage, cornichons, oignons sur un pain de graines de sésame. Chris
I haven't found where they started expressing diameters in mm but I have found where weights were expressed in grams in the coinage Act of 1873 with the half dollar being set at "12 and 1/2 grams with the quarters and dimes being in proportion." The government authorized the use of the metric system throughout the country in 1866 (Metric act of 1866 (Public Law 39-183)) This authorized its use but didn't make its use mandatory (mistake).
