Diameter 101The diameter of ancient coins is usually expressed in millimeters. In Forum’s catalog descriptions you will usually find the coin’s weight, then maximum diameter, followed by the die axis. The ancient denomination names and values of some late Roman bronze coins are uncertain. Often they are referred to 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.
We also do not know the names of the denomination or value of many Greek and Roman provincial coins often the denomination given is simply AE, indicating a base metal or alloy, mostly copper, followed by the diameter in millimeters. An AE23 is a base metal (copper, bronze or brass) coin with a 23 millimeter diameter.
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 mmInch and Millimeter EquivalentsDivide millimeter lengths by 25.4 to determine the inch length.MMApproximate Size In InchesExact Size In Inches1mm1/32 Inch0.03937 Inches2mm1/16 Inch0.07874 Inches3mm3/32 Inch0.11811 Inches4mm1/8 Inch0.15748 Inches5mm3/16 Inch0.19685 Inches6mmAlmost 1/4 Inch0.23622 Inches7mmLittle over 1/4 Inch0.27559 Inches8mm5/16 Inch0.31496 Inches9mmAlmost 3/8 Inch0.35433 Inches10mmLittle over 3/8 Inch0.39370 Inches11mm7/16 Inch0.43307 Inches12mmAlmost 1/2 Inch0.47244 Inches13mmLittle over 1/2 Inch0.51181 Inches14mm9/16 Inch0.55118 Inches15mmAlmost 5/8 Inch0.59055 Inches16mm5/8 Inch0.62992 Inches17mmAlmost 11/16 Inch0.66929 Inches18mmAlmost 3/4 Inch0.70866 Inches19mmLittle over 3/4 Inch0.74803 Inches20mmAlmost 13/16 Inch0.78740 Inches21mmLittle over 13/16 Inch0.82677 Inches22mmAlmost 7/8 Inch0.86614 Inches23mmLittle over 7/8 Inch0.90551 Inches24mm15/16 Inch0.94488 Inches25mm1 Inch0.98425 Inches
- 101Ancient| |Coin Collecting| 101
Uncleaned Ancient Coins 101
Roman Coin Attribution 101
Greek Coin Collecting 101
Patina| 101
Magnification| 101
Die Alignment| 101
Bust| |Types| 101