Coin Weight Check

The coin weight check is one of the fundamental and particularly accessible methods of coin authenticity checking. Since every official precious-metal coin is struck to exact statutory specifications, comparing the actual weight with the published target weight yields an important indication of authenticity or forgery. The method requires only a precise fine scale and the corresponding reference values – it is therefore feasible for any private investor without laboratory equipment.

Basic principle and significance

Every state mint – from the Austrian Mint to the Royal Canadian Mint – publishes binding technical data for its bullion coins: gross weight (total weight of the coin), fineness and, resulting from these, the fine weight (weight of the pure metal contained). The gross weight of a coin results from the fine weight plus the alloying metals.

The basic principle can be expressed in a simple formula:

Fine weight = gross weight × fineness (as a decimal)

Example Krugerrand 1 oz:
Gross weight = 33.930 g
Fineness     = 916/1000 = 0.9167 (22 carat)
Fine weight  = 33.930 g × 0.9167 ≈ 31.103 g (= 1 troy ounce of gold)

A genuine coin must lie within the manufacturing tolerance stated by the producer. If the measured weight lies outside this tolerance, there is reasonable suspicion of a forgery or a damaged coin.

Known target weights of important bullion coins

Detailed values for further denominations and coin types are provided by the coin weight checker.

Procedure: step by step

1. Calibrate the scale: use a precision scale with a resolution of at least 0.001 g (1 mg) and check it against a certified calibration weight. Inexpensive fine scales for home use are available from around 15–30 euros and are sufficient for this check.
2. Coin dry and clean: residues of fingerprints, grease or moisture can minimally distort the measurement result. Handle the coin briefly with a clean cloth or leather pad.
3. Weigh: place the coin centrally on the weighing surface and read off the value. Ideally repeat two or three times to rule out measurement deviations.
4. Compare: match the measured weight against the official target weight. If the deviation lies within the manufacturer's tolerance (typically ± 0.05 g for modern bullion coins), the weight criterion is met.
5. Interpret the result: a passed weight check is a necessary but not a sufficient authenticity criterion. Professional counterfeiters know the target weights and produce corresponding forgeries (e.g. with a tungsten core).

Limits of the method and the tungsten problem

The greatest weakness of the pure weight check lies in the tungsten counterfeit: tungsten possesses, at 19.30 g/cm³, almost the same density as gold (19.32 g/cm³). A coin with a tungsten core and a thin gold layer can show the same weight as a genuine coin and thus passes the weight check, yet is worthless.

The weight check should therefore always be combined with:

• the coin dimensions check (diameter and thickness)
• the density measurement by Archimedes (reliably exposes tungsten counterfeits)
• the magnet test (precious metals are not ferromagnetic)
• the ping test for silver coins
• if suspicious: the X-ray fluorescence analysis (XRF)

For a complete testing routine the coin authenticity checker is recommended, which evaluates all relevant parameters in combination.

Tolerances and statutory provisions

State mints are legally obliged to manufacture their coins within tight production tolerances. These tolerance limits, known as the "remedy", vary by coin type and issue period. For modern bullion coins the following guide values generally apply:

• Gold coins: ± 0.05 g (for 1 oz coins this corresponds to approx. 0.15%)
• Silver coins: ± 0.10 g (for 1 oz coins approx. 0.32%)
• Historical circulation coins: up to ± 0.30 g depending on the striking period

Older circulation coins from the 19th and early 20th centuries – such as 20-mark gold coins or sovereigns – can lie slightly below the target weight due to natural circulation wear. Here a fine touch in the valuation is called for.

Practical tips for investors

• Use the melt value calculator to determine the current material value of the coin from the measured gross weight and the known fineness.
• For frequent checks, a high-quality calibration weight set (e.g. OIML class M1 or better), regularly checked against the scale, is worthwhile.
• Never place coins directly on metal surfaces; a plastic or leather pad prevents scratches and minimises electrostatic influences.
• Document results: anyone who buys regularly benefits from a simple table with date, coin type, target weight and measured value.

In brief

The coin weight check is an indispensable first step in any authenticity control of precious-metal coins – quick, inexpensive and without chemicals. It reliably rules out coarse forgeries but should always be supplemented by further methods such as size measurement and density determination in order to also reliably detect sophisticated tungsten counterfeits.