Coin Weight Check

The coin weight check is one of the most fundamental and readily accessible methods of coin authenticity testing. 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 counterfeiting. The method requires only a precise fine balance 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, 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 falls outside this tolerance, there is well-founded suspicion of a counterfeit or a damaged coin.

Known target weights of important bullion coins

Detailed figures for further denominations and coin types are provided by the Coin Weight Checker.

Procedure: step by step

1. Calibrate the scale: use a precision balance with a resolution of at least 0.001 g (1 mg) and check it against a certified calibration weight. Inexpensive fine balances for home use are available from around GBP 15-30 and are sufficient for this test.
2. Coin dry and clean: residues of fingerprints, grease or moisture can slightly distort the measurement. Handle the coin briefly with a clean cloth or leather pad.
3. Weigh: place the coin centrally on the weighing surface and read the value. Ideally repeat two or three times to rule out measurement deviations.
4. Compare: compare the measured weight with 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 fakes accordingly (e.g. with a tungsten core).

Limits of the method and the tungsten problem

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

The weight check should therefore always be combined with:

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

For a complete testing routine the Coin Authenticity Check is recommended, which evaluates all relevant parameters together.

Tolerances and statutory regulations

State mints are legally obliged to produce their coins within narrow manufacturing tolerances. These tolerance limits, known as the "remedy", vary by coin type and issuing country. 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 circulating coins: up to +/- 0.30 g depending on the striking period

Older circulating coins from the 19th and early 20th centuries - such as 20-Mark gold coins or Sovereigns - may lie slightly below the target weight due to natural wear in circulation. Here, a careful judgement is required.

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 the 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 - fast, low-cost and chemical-free. It reliably rules out crude counterfeits, but should always be supplemented by further methods such as size measurement and density determination in order to reliably detect even sophisticated tungsten counterfeits.