Coin Dimensions Verification (Diameter / Thickness)
Also: Dimensional check, Measurement check, Size test
Coin dimensions verification measures a coin's diameter and thickness with a calliper or gauge and compares the readings against the official striking specifications, exposing forgeries by their deviant measurements.
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Every officially struck bullion coin is made to exact striking specifications: diameter and thickness are fixed to tenths of a millimetre and form part of the legal coin description. Because forgers often use other metals — tungsten above all in gold coins — it is frequently impossible to hit the correct dimensions with a cheap base metal at the same time. Coin dimensions verification is therefore one of the fastest and cheapest methods of authenticity testing.
Why measurements expose forgeries
A coin with a tungsten core and gold plating can simulate the correct weight, because tungsten (19.25 g/cm³) comes very close to the density of gold (19.32 g/cm³). The geometry, however, often does not match: a tungsten blank cut to the exact weight deviates in thickness at the same diameter — or vice versa. Even the most sophisticated tungsten forgeries can be identified with high confidence through the combined check of weight and dimension.
How to test
- Get the tool ready — a digital calliper with at least 0.01 mm resolution is enough. Hardware-store callipers from around EUR 15 are suitable; for regular checks, models with hardened steel are recommended.
- Clean the coin — dirt or fingerprints can distort readings by 0.1–0.3 mm.
- Measure the diameter — measure the coin along at least two axes (0° and 90°) to detect any ovality.
- Measure the thickness — measure at the centre of the coin, not at the edge (edge lettering can interfere).
- Compare — match the measured values against the maker's specification; deviations over ±0.2 mm are a warning sign.
Reference dimensions of common gold coins
| Coin | Denomination | Diameter | Thickness |
|---|---|---|---|
| Krugerrand | 1 oz | 32.77 mm | 2.84 mm |
| Vienna Philharmonic | 1 oz | 37.00 mm | 2.00 mm |
| Maple Leaf | 1 oz | 30.00 mm | 2.87 mm |
| American Eagle | 1 oz | 32.70 mm | 2.87 mm |
| Britannia | 1 oz | 32.69 mm | 2.79 mm |
The values apply to current striking years; older years may deviate slightly. Always verify official data with the respective maker.
Formula: volume as a plausibility check
Knowing the diameter and thickness, you can compute the theoretical volume and from it — combined with the measured weight — determine the density:
V (cm³) = π × (diameter / 2)² × thickness [all measures in cm]
Density (g/cm³) = weight (g) / V (cm³)
For a genuine 1-oz gold coin (31.10 g) the result is a density close to 19.3 g/cm³. If the value deviates markedly, forgery is suspected. Use the melt value calculator and the coin weight checker as supporting tools.
Limits of the method
The dimension check is no cure-all. High-quality forgeries with a matching alloy mix can show correct measurements. A conclusively safe verdict comes only from the Archimedes density test or X-ray fluorescence analysis (XRF). Even so, the dimension check is an indispensable first filter: it is non-destructive, takes seconds, and immediately eliminates a large share of simple forgeries.
In brief
Apply a digital calliper, measure diameter and thickness along two axes, and match against the official maker specification — deviations over ±0.2 mm are a serious warning sign and justify a deeper check via a weight test and authenticity testing.