Coin Dimensions Check (Diameter/Thickness)
Also: Dimensional check, Measurement verification, Size test
The coin dimensions check measures the diameter and thickness of a coin with callipers or a gauge and compares the values against the official minting specifications, exposing forgeries through deviating measurements.
Every officially struck bullion coin is manufactured to exact minting specifications: diameter and thickness are defined to the tenth of a millimetre and form part of the statutory coin description. Because forgers frequently use other metals — above all tungsten in gold coins — the correct dimensions often cannot be matched at the same time using a cheap base metal. The coin dimensions check is therefore one of the fastest and cheapest methods of authenticity testing.
Why dimensions expose forgeries
A coin made from a tungsten core with gold plating can simulate the correct weight, because tungsten (19.25 g/cm³) comes very close to gold's density (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 modern tungsten forgeries can be identified with high confidence through the combined check of weight and dimensions.
How it is tested
- Prepare the tool – Digital callipers with at least 0.01 mm resolution are sufficient. Callipers from the hardware store from about €15 are suitable; for regular testing, models with hardened steel are recommended.
- Clean the coin – Dirt or fingerprints can falsify the reading by 0.1–0.3 mm.
- Measure the diameter – Measure the coin along at least two axes (0° and 90°) to detect ovality.
- Measure the thickness – Measure at the centre of the coin, not at the rim (edge lettering can interfere).
- Compare – Compare the measured values with the manufacturer's specification; deviations of more than ±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 minting years; older years may deviate slightly. Always verify official data with the respective manufacturer.
Formula: volume as a plausibility check
Anyone who knows the diameter and thickness can calculate the theoretical volume and, combined with the measured weight, determine the density:
V (cm³) = π × (Diameter / 2)² × Thickness [all dimensions in cm]
Density (g/cm³) = Weight (g) / V (cm³)
For a genuine 1-oz gold coin (31.10 g) this gives a density close to 19.3 g/cm³. If the value deviates significantly, forgery is suspected. Use the Melt Value Calculator and the Coin Weight Checker as a complement.
Limits of the method
The dimensional check is no panacea. High-quality forgeries with a matching alloy mix can display correct dimensions. A conclusively certain statement is only provided by density measurement using the Archimedes principle or X-ray fluorescence analysis (XRF). The dimensional check is, however, an indispensable first filter: it is non-destructive, takes seconds and immediately eliminates a large proportion of simple forgeries.
In brief
Apply digital callipers, measure diameter and thickness along two axes, compare with the official manufacturer's specification — deviations of more than ±0.2 mm are a serious warning sign and justify a deeper check via weight test and authenticity test.