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Diameter 101The Diameter of ancient coins is usually expressed in millimeters. In FORVM’s catalog descriptions you will usually find the coin’s weight, then diameter, followed by the die axis. For some Roman coins the ancient name of the denominations are unknown.  We don't know the names or values of the late Roman bronze coins and refer to them with the following size (diameter) designations:
AE1 = over 25mm
AE2 = 21mm – 25mm
AE3 = 17mm – 21mm
AE4 = less than 17mm 

The denomination of a coin is based on the usual size for the type.  The coins were handmade and the same type can vary significantly in size.  Some individual coins that are larger or smaller than usual for their type will be outside the parameters above. Diameters of Modern Coins for ComparisonSince many Americans are not familiar with millimeters and because it can help anyone visualize size better, we provide comparisons to modern coins below.U.S. Coins Euro CoinsU.S. Half dollar30.5 mm 2 euro25.75 mmU.S. Quarter  24.1 mm 1 euro23.25 mmU.S. Dime    17.9 mm 50 eurocent24.25 mmU.S. Nickel 21.3 mm 20 eurocent22.25 mmU.S. Penny   18.9 mm 10 eurocent19.75 mm   5 eurocent21.75 mm   2 Eurocent18.75 mm   1 Eurocent16.25 mmU.K. Coins    2 Pounds28.4 mm   1 Pound22.5 mm   Fifty pence27.3 mm (7 sided)   Twenty Pence21.4 mm (7 sided)   Ten Pence24.5 mm   Five pence18 mm   Two pence 26 mm   One pence20 mm
Diameter 101The Diameter of ancient coins is usually expressed in millimeters. In FORVM’s catalog descriptions you will usually find the coin’s weight, then diameter, followed by the die axis. For some Roman coins the ancient name of the denominations are unknown.  We don't know the names or values of the late Roman bronze coins and refer to them with the following size (diameter) designations:
AE1 = over 25mm
AE2 = 21mm – 25mm
AE3 = 17mm – 21mm
AE4 = less than 17mm 

The denomination of a coin is based on the usual size for the type.  The coins were handmade and the same type can vary significantly in size.  Some individual coins that are larger or smaller than usual for their type will be outside the parameters above. Diameters of Modern Coins for ComparisonSince many Americans are not familiar with millimeters and because it can help anyone visualize size better, we provide comparisons to modern coins below.U.S. Coins Euro CoinsU.S. Half dollar30.5 mm 2 euro25.75 mmU.S. Quarter  24.1 mm 1 euro23.25 mmU.S. Dime    17.9 mm 50 eurocent24.25 mmU.S. Nickel 21.3 mm 20 eurocent22.25 mmU.S. Penny   18.9 mm 10 eurocent19.75 mm   5 eurocent21.75 mm   2 Eurocent18.75 mm   1 Eurocent16.25 mmU.K. Coins    2 Pounds28.4 mm   1 Pound22.5 mm   Fifty pence27.3 mm (7 sided)   Twenty Pence21.4 mm (7 sided)   Ten Pence24.5 mm   Five pence18 mm   Two pence 26 mm   One pence20 mm