Why Is a Dime Smaller Than a Penny? the Surprising Secret Behind Us Coin Sizes
While the coin originally featured Lady Liberty across various iterations, the modern Roosevelt dime debuted in 1946 to honor President Franklin D. Roosevelt and his leadership of the March of Dimes polio campaign. Chief Engraver John R. Sinnock preserved the traditional proportions established by nineteenth-century silver issues, solidifying the measurements still struck by the billions in Philadelphia and Denver.
Official US Mint coin specifications dictate rigid dimensional tolerances for the denomination:
- Diameter: Exactly 17.91 mm (0.705 inches).
- Thickness: 1.35 mm (0.053 inches).
- Gross Weight: 2.268 grams (0.080 ounces).
- Edge Milling: Exactly 118 reeded grooves.
The ridged edge was not created for decoration. On historic 90% silver coinage, reeding was a security countermeasure designed to prevent "clipping" or "shaving", the illicit practice of scraping precious metal dust off the rim before spending the lightened coin at face value. While modern circulation coins no longer contain silver, the 118 reeds remain. They serve as tactile aids for visually impaired citizens and allow automated coin sorting machines to verify legitimate pieces through optical edge sensors.
In 1965, surging silver spot prices forced Congress to pass the landmark Coinage Act of 1965, eliminating silver from circulating dimes and quarters. Metallurgists replaced the 90% silver alloy with a cupronickel clad composition consisting of an outer jacket of 75% copper and 25% nickel bonded to a core of pure copper. The total metallurgy balances out to 91.67% copper and 8.33% nickel. Despite this radical metallurgical transformation, the 17.91 mm profile remained untouched.
