Using the Coin Caliper to Identify Dirty or Worn Coins

You pull a coin out of the ground and it's encrusted, green, and completely unreadable. No date, no face, no denomination. But it still has a measurable diameter, and that diameter narrows down what it could be. The Coin Caliper uses this approach to give you a shortlist of matches right in the field.

How It Works

• Open the Coin Caliper tool from the More tab
• Place the coin flat on your phone screen
• Adjust the slider until the on-screen circle matches the coin's edge
• The app reads the diameter and searches its database for coins within 0.5mm of that size

Matching Results

After measuring, you'll see a list of possible matches. Each result shows the coin name, denomination, country, era, and its official diameter. If a coin is 25.0mm, you might see matches for a UK shilling (23.5mm), a US quarter (24.3mm), and several others close to that range. The closest matches appear first.

Filtering by Region and Era

If you know roughly where you're detecting (and you do), filter the results by region. Detecting in England? Filter to UK coins and the list shrinks dramatically. If you also know the site is medieval, add an era filter and you might be down to two or three possibilities. The fewer candidates, the easier the ID.

Calibrating for Your Screen

Different phones have different screen sizes and pixel densities. The first time you use the Coin Caliper, calibrate it using a coin you already know — a modern coin from your pocket works perfectly. Place it on screen, adjust to match, and confirm the known diameter. This ensures measurements are accurate on your specific device.

Tips for Accurate Measurements

• Clean off loose dirt first — a crust of soil adds millimetres
• Measure the widest point — worn coins can be slightly irregular
• If the coin is heavily corroded, account for material loss — it might be slightly smaller than the original