What it does
A genuine coin of a given type is made to a published weight and size. If yours is far off on a careful scale and callipers, that is a reason to look closer. This tool lines up what you measured against the official figures and shows the difference in grams, millimetres and per cent, with a pill that says whether it is inside the allowance you set.
It never says a coin is real or fake. Worn coins lose weight, cleaned or damaged ones can change size, and mints allow a small spread. If a figure is far off, ask an expert or a dealer to check it. After weighing, try specific gravity for a density check, or melt value for what the metal is worth.
How to use it
- Pick a coin from the list, or choose “My own figures” and type the official weight and diameter.
- Weigh the coin in grams and measure its width in millimetres. Type both. Add thickness if you have it.
- Read the pills. Change the allowances if you want something tighter or looser.
What each setting does
- Official weight, diameter, thickness: the mint’s figures for that coin type.
- Metal: only used with thickness, to work out the weight of a smooth disc of that size.
- Weight allowance: per cent difference you accept. Starts at 1%.
- Diameter allowance: starts at 0.5%. Mints publish their own tolerances, so check theirs.
Worked example
A Morgan dollar should weigh 26.73 g and be 38.10 mm wide. Yours weighs 26.20 g and measures 38.10 mm. The weight is 0.53 g lighter, which is -1.98%, outside the 1% allowance. The width is exactly right (0.00%). A worn coin can easily lose that much, but far off means ask an expert.
FAQs
Why is my old coin lighter? Wear removes metal. Many circulated coins sit a little under the official weight.
Does a pass mean it’s genuine? No. Good fakes can match weight and size. It is one check among several.
Why is thickness optional? It is harder to measure well. With it and a metal chosen, you also see the weight of a smooth disc of that size as a rough guide.
Is anything sent anywhere? No. Everything is worked out in your browser.
Where the numbers come from
- Coin weights and diameters: the issuing mints' published specifications (US Mint, Royal Mint, Royal Canadian Mint, South African Mint, India Government Mint). Check your coin against its own mint's page.
- Densities of the metals are general physical values. Drawings and wording are fibub's own.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-10.