Why a scale and a magnet beat guesswork

Counterfeit coins that turn up in UK change are rarely masterpieces. They are made to survive a glance and a quick feel, because that is all most of us give them. That is precisely why they fall apart under two very simple tests: an accurate weight and a magnet. A pocket digital scale reading to 0.01 g costs very little, and a strong neodymium magnet costs even less. Together they will catch the great majority of the duds that slip into your pocket, a charity tin or a car boot haul.

This will not replace expert authentication for a genuinely valuable piece, but for everyday sorting it is remarkably effective.

Start with the numbers: weight and diameter

Every UK coin has a published specification. Weighing a suspect and comparing it with a known genuine example of the same denomination is the single most useful home test. Use a scale that reads to 0.01 g; kitchen scales are far too coarse.

• 1p — 3.56 g, 20.3 mm
• 2p — 7.12 g, 25.9 mm
• 5p — 3.25 g, 18.0 mm
• 10p — 6.50 g, 24.5 mm
• 20p — 5.00 g, 21.4 mm
• 50p — 8.00 g, 27.3 mm
• Round £1 (1983–2016) — 9.50 g, 22.5 mm
• 12-sided £1 (2017 onwards) — 8.75 g, 23.43 mm
• £2 — 12.00 g, 28.4 mm

Allow for wear. A coin that has circulated for thirty years can be a few per cent light, so treat 0.1–0.2 g of drift as normal on a worn piece and a discrepancy of half a gram as a serious question mark. Check diameter too — laying a genuine coin over the suspect one will expose an obvious mismatch instantly, no callipers required.

The magnet test: what should stick and what should not

UK coins come from a small family of alloys: bronze, cupro-nickel, nickel-brass and copper-plated steel. Only one of those is magnetic, which gives you a very clean test.

• 1p and 2p dated 1992 onwards are copper-plated steel and should be attracted to a magnet. If a modern penny ignores a strong magnet, something is wrong.
• 1p and 2p dated before 1992 are bronze and should show no attraction at all. A pre-1992 penny that leaps onto your magnet is almost certainly a fake or a plated novelty.
• 5p, 10p, 20p, 50p, £2 and the round £1 are cupro-nickel or nickel-brass. They should show no meaningful pull. A silver-coloured coin that sticks firmly is a red flag.

The 12-sided £1 deserves a note: its inner disc carries a nickel plating that can give a very faint tug. What you are watching for is a firm, unmistakable grab, not a whisper.

The slide test: eddy currents on a tilt

Here is a test that feels like a party trick and works beautifully. Lay a coin face up and tilt it very slightly, then place a small strong magnet near the top edge and let it slide down the face. On a genuine cupro-nickel or nickel-brass coin the magnet slows noticeably, because it induces circulating eddy currents in the metal that resist its movement. On a plated steel fake the magnet grips and stops dead. On a plated zinc or lead fake it slithers down as if the coin were plastic. Try it on a known genuine 50p first so you learn how the resistance should feel.

Feel, ring and edge

Your hands already know more than you think. Genuine UK coins have a crisp milled or plain edge, sharp lettering and clean, even relief. Fakes tend to be slightly soft, blobby or grainy, often with a seam or a faint tooling mark near the rim. Run a fingernail around the edge: a rough ridge where two halves of a mould met is a strong warning sign.

The ring test is quick and surprisingly useful. Balance a coin on the tip of one finger and strike it gently with another coin of the same size. A genuine coin gives a clear, sustained ring; a cast fake, especially in a lead-based alloy, gives a dull thud. It works best on the larger coins and it takes practice, but it costs nothing.

Building a reference set — and knowing when to stop

Keep a small tin of known genuine coins: one of each denomination you handle often, ideally in decent condition. Weigh them once, write the figures on a card, and you have a permanent reference bench with no guesswork. Weigh the suspect against the reference, check the magnet, then check the slide. Three tests, under a minute.

If a coin fails one test but passes the others, be curious rather than certain — worn or damaged genuine coins can behave oddly. If it fails two, set it aside and look more closely. And if you think you have found something genuinely unusual, remember that a fake is not the only possibility: off-metal strikes, trial pieces and mint errors all exist, and those deserve a proper expert eye rather than a magnet.