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Identify a UK Coin by Diameter
Got an unidentified British coin and a ruler or callipers? Enter the diameter in mm below and we’ll instantly match it against 4,700+ UK coins from 1066 to 2026. Most denominations are unique to within 0.1 mm, so diameter alone usually narrows it to one or two candidates. Companion to our weight identifier — pick whichever measurement tool you have. Free, no sign-up.
Common UK coin diameter reference
Quick-reference table of typical UK coin diameters. For the heptagonal 20p and 50p the value is the flat-to-flat dimension across opposite parallel edges — that’s what a calliper naturally measures and what the Royal Mint publishes.
| Coin | Diameter | Composition |
|---|---|---|
| 1p (decimal, 1971+) | 20.32 mm | Bronze / copper-plated steel |
| 2p (decimal, 1971+) | 25.91 mm | Bronze / copper-plated steel |
| Modern 5p (2012+) | 18.00 mm | Nickel-plated steel |
| Pre-2012 5p | 18.00 mm | Cupronickel |
| Modern 10p (2012+) | 24.50 mm | Nickel-plated steel |
| Pre-2012 10p | 24.50 mm | Cupronickel |
| 20p (heptagonal, flat-to-flat) | 21.40 mm | Cupronickel |
| Modern 50p (1997+) | 27.30 mm | Cupronickel |
| Pre-1997 large 50p | 30.00 mm | Cupronickel |
| £1 round (1983–2017) | 22.50 mm | Nickel-brass |
| £1 12-sided (2017+) | 23.43 mm | Bimetal |
| £2 bimetal (1997+) | 28.40 mm | Cupronickel / nickel-brass |
| Sovereign | 22.05 mm | 22 carat gold |
| Half sovereign | 19.30 mm | 22 carat gold |
| Britannia 1 oz silver | 38.61 mm | 999 silver |
| Britannia 1 oz gold | 32.69 mm | 9999 gold (post-2013) |
| Crown (1816–present) | 38.61 mm | Silver / cupronickel |
| Half crown (pre-1947) | 32.31 mm | Sterling / .500 silver |
| Florin (pre-1947) | 28.50 mm | Sterling / .500 silver |
| Shilling (pre-1947) | 23.59 mm | Sterling / .500 silver |
| Sixpence (pre-1947) | 19.30 mm | Sterling / .500 silver |
| Threepence (Brass, 1937–67) | 21.00 mm | Nickel-brass |
| Farthing (1860–1956) | 20.19 mm | Bronze |
Tips for accurate measurement
- Use digital callipers (around £8–15 on Amazon) for sub-millimetre precision. A steel ruler is fine for ID-level accuracy if you eyeball ±0.5 mm.
- Heptagonal coins (20p, 50p) are measured flat-to-flat — across opposite parallel edges, not corner-to-corner. Corner-to-corner reads 1–2 mm larger than the catalogue figure.
- Bimetal coins (£2, 12-sided £1) are measured at the outer ring. The inner disc dimension is published separately and isn’t what we match against.
- Worn or filed coins read smaller than mint spec. Edge wear can knock 0.2–0.5 mm off a heavily-circulated pre-decimal silver coin.
- Cross-check with weight. If diameter narrows to two candidates, the weight identifier almost always picks the winner.
Other identification tools
- Coin weight identifier — the companion tool, recommended when you have a scale
- Decision-tree coin identifier — better when you have a clear date and design
- Silver melt calculator
- UK coin grading guide
- Coin errors catalogue