What are official weight and dimension specifications for U.S. silver dimes (pre‑1965) and how much variation is acceptable?
Executive summary
Pre‑1965 U.S. dimes (commonly called “90% silver” dimes) were specified by the U.S. Mint to be struck in a 90% silver / 10% copper alloy, with a standard total weight of 2.50 grams and a diameter of 17.9 millimeters (0.705 inch) — facts consistently cited across numismatic and bullion sources [1][2][3]. What the provided reporting does not include is an official published manufacturing tolerance for acceptable weight or dimensional variation from the Mint; available dealer and testing guidance therefore becomes the practical fallback for detecting outliers and counterfeits [4].
1. The official metal recipe and what it means in grams
The metal composition recorded for circulating U.S. dimes minted through 1964 is 90% silver and 10% copper; with a total coin mass of 2.50 grams that works out to roughly 2.25 grams of pure silver per coin (90% × 2.50 g) as shown in multiple references [1][5][3]. Commercial bullion sites translate that silver mass to troy‑ounce figures for investors: Money Metals quotes a silver content of about 0.0715 troy ounces per dime (a commonly used industry figure), while other calculators produce numbers near 0.0723 troy ounces — differences due to rounding and unit conversion conventions in the secondary market [6][5].
2. Size, edge and design dimensions recorded by reference sources
Standard diameter for these pre‑1965 dimes is listed at 17.9 millimeters (0.705 inch) with the familiar reeded edge that helps verification and weight distribution, and numismatic guides consistently record the 2.50‑gram total mass across Roosevelt, Mercury and Barber designs up through 1964 [2][7][3]. Several sources also note that post‑1964 dimes changed composition and mass (to about 2.268 g and a copper‑nickel clad construction), which underscores why date is a primary indicator when identifying silver dimes [8][9].
3. What “acceptable variation” the reporting actually documents — and the reporting gap
None of the provided sources quote an explicit U.S. Mint manufacturing tolerance table for weight or diameter for pre‑1965 dimes; the public references and dealer guides therefore stop short of asserting a single official allowance for variation and instead rely on practical checks and averages used by dealers and investors [1][4]. As a result, authoritative tolerance figures — as a legal manufacturing spec — are not present in the supplied reporting; that omission prevents a definitive, source‑backed statement about exact allowable grams or millimeter deviation from nominal.
4. How the market and dealers treat deviations in practice
Where official tolerances are absent from these sources, industry practice as reported is to flag any dime that noticeably deviates from the 2.50‑gram expectation or that reacts to a magnet as suspicious; tools like specific‑gravity testing, precise digital scales, diameter gauges, and magnet checks are recommended to confirm authenticity and metal content [4][10]. Secondary sources frame these coins as “junk silver” for bullion purposes — traded for metal value rather than numismatic grade — so small circulation wear that lowers mass somewhat is expected, but sharp departures from 2.50 g or from the 17.9 mm diameter typically trigger further scrutiny [7][10].
5. What readers should take away and where the evidence ends
Definitively: pre‑1965 U.S. dimes are 90% silver / 10% copper, 2.50 g total weight, 17.9 mm diameter, and have a reeded edge, according to the Mint history and consistent industry references [1][2][3]. Indeterminately: the supplied reporting does not contain the U.S. Mint’s formal manufacturing tolerances (weight/diameter tolerances), so any precise “acceptable variation” limits must be sought in Mint archival specifications or technical production documents not included here; until those are produced, dealers’ practical thresholds and metallurgical tests (specific gravity, precise weighing, magnet response) are the operational standard cited by the sources for spotting outliers and counterfeits [4][6].