Identify a UK Coin by Diameter
A 1p is 20.3 mm across, a 5p 18 mm, a 10p 24.5 mm, a 20p 21.4 mm, a 50p 27.3 mm, a £1 23.43 mm and a £2 28.4 mm (full size and weight table). Got an unidentified British coin and a ruler or callipers? Enter the diameter in mm below and we’ll instantly match it against every UK coin in our catalogue, 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.
Tip: got scales too? The full coin identifier combines diameter, weight and shape in one search, which narrows worn coins much harder than diameter alone.
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. For thickness and weight as well, see UK coin sizes and weights.
| Coin | Diameter | Composition |
|---|---|---|
| Copper-plated steel 1p (1992 to now) | 20.32 mm | Copper-plated steel |
| Bronze 1p (1971 to 1991) | 20.32 mm | Bronze |
| Bronze penny (1860 to 1970) | 30.86 mm | Bronze |
| Copper-plated steel 2p (1992 to now) | 25.91 mm | Copper-plated steel |
| Bronze 2p (1971 to 1991) | 25.91 mm | Bronze |
| Nickel-plated steel 5p (2012 to now) | 18.00 mm | Nickel-plated steel |
| Small 5p (1990 to 2011) | 18.00 mm | Cupro-nickel |
| Large 5p (1968 to 1990) | 23.59 mm | Cupro-nickel |
| Nickel-plated steel 10p (2012 to now) | 24.50 mm | Nickel-plated steel |
| Small 10p (1992 to 2011) | 24.50 mm | Cupro-nickel |
| Large 10p (1968 to 1992) | 28.50 mm | Cupro-nickel |
| 20p (1982 to now) | 21.40 mm | Cupro-nickel |
| Small 50p (1997 to now) | 27.30 mm | Cupro-nickel |
| Large 50p (1969 to 1997) | 30.00 mm | Cupro-nickel |
| 12-sided £1 (2017 to now) | 23.43 mm | Nickel-brass outer, nickel-plated alloy inner |
| Round £1 (1983 to 2016) | 22.50 mm | Nickel-brass |
| Bimetallic £2 (1997 to now) | 28.40 mm | Nickel-brass outer, cupro-nickel inner |
| Nickel-brass £2 (1986 to 1996) | 28.40 mm | Nickel-brass |
| Decimal crown (25p) (1972 to 1981) | 38.61 mm | Cupro-nickel |
| £5 crown (1990 to now) | 38.61 mm | Cupro-nickel |
| Sovereign (1817 to now) | 22.05 mm | 22-carat gold |
| Half sovereign (1817 to now) | 19.30 mm | 22-carat gold |
| Quarter sovereign (2009 to now) | 13.50 mm | 22-carat gold |
| Crown (1816 to 1970) | 38.61 mm | Silver to 1946, cupro-nickel from 1947 |
| Half crown (1816 to 1970) | 32.31 mm | Silver to 1946, cupro-nickel from 1947 |
| Florin (1849 to 1970) | 28.50 mm | Silver to 1946, cupro-nickel from 1947 |
| Shilling (1816 to 1970) | 23.60 mm | Silver to 1946, cupro-nickel from 1947 |
| Sixpence (1816 to 1970) | 19.41 mm | Silver to 1946, cupro-nickel from 1947 |
| Brass threepence (1937 to 1970) | 21.80 mm | Nickel-brass |
| Silver threepence (1816 to 1944) | 16.26 mm | Silver |
| Bronze halfpenny (1860 to 1970) | 25.48 mm | Bronze |
| Bronze farthing (1860 to 1970) | 20.19 mm | Bronze |
| Britannia 1 oz silver | 38.61 mm | 999 silver (Royal Mint bullion specification) |
| Britannia 1 oz gold | 32.69 mm | 9999 gold (Royal Mint bullion specification) |
Tips for accurate measurement
- Use digital callipers for sub-millimetre precision, they are inexpensive. 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