Coin Size Check (Diameter/Thickness)
Also: Dimensional check, Measurement control, Size test
The coin size check measures a coin's diameter and thickness with callipers or a gauge and compares the values with the official minting specifications, in order to expose counterfeits by their deviating dimensions.
Every officially struck bullion coin is produced to exact minting specifications: diameter and thickness are fixed to a tenth of a millimetre and form part of the statutory coin description. Because counterfeiters frequently use other metals - above all tungsten in gold coins - correct dimensions often cannot be achieved simultaneously with a cheap base metal. The coin size check is therefore one of the fastest and cheapest methods of authenticity testing.
Why dimensions expose counterfeits
A coin with a tungsten core and a 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, is often wrong: a tungsten blank cut to the exact weight deviates in thickness at the same diameter - or vice versa. Even the most modern tungsten counterfeits can be identified with high certainty by the combined check of weight and dimensions.
How the check is done
- Prepare the tool - digital callipers with a resolution of at least 0.01 mm are sufficient. Callipers from the hardware store from around GBP 15 are suitable; for regular checks, hardened-steel models are recommended.
- Clean the coin - dirt or fingerprints can distort the reading by 0.1-0.3 mm.
- Measure the diameter - measure the coin on at least two axes (0 degrees and 90 degrees) to detect ovality.
- Measure the thickness - measure in the centre of the coin, not at the edge (edge lettering can interfere).
- Compare - compare the measured values with the manufacturer's specification; deviations above +/- 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 mintage years; older years may differ 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 yields a density close to 19.3 g/cm3. If the value deviates significantly, there is grounds to suspect a counterfeit. Use the Melt Value Calculator and the Coin Weight Checker as complements.
Limits of the method
The dimensional check is no cure-all. High-quality counterfeits with a matching alloy mix can display correct dimensions. A conclusively reliable statement is only provided by the density measurement by the Archimedes principle or by X-ray fluorescence analysis (XRF). The dimensional check is, however, an indispensable first filter: it is non-destructive, takes seconds and immediately weeds out a large proportion of simple counterfeits.
In brief
Apply digital callipers, measure diameter and thickness on two axes, compare with the official manufacturer's specification - deviations above +/- 0.2 mm are a serious warning sign and justify a deeper check by weight test and authenticity testing.