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Diameter 101

The Diameter of ancient coins is usually expressed in millimeters. In FORVM’s catalog descriptions you will usually find the coin’s weight, then maximum diameter, followed by the die axis.

We don't know the ancient denomination 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 Comparison

Since 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 Coins

U.S. Half dollar30.5 mm 2 euro25.75 mm
U.S. Quarter  24.1 mm 1 euro23.25 mm
U.S. Dime    17.9 mm 50 eurocent24.25 mm
U.S. Nickel 21.3 mm 20 eurocent22.25 mm
U.S. Penny   18.9 mm 10 eurocent19.75 mm
   5 eurocent21.75 mm
   2 Eurocent18.75 mm
   1 Eurocent16.25 mm

U.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 101

The Diameter of ancient coins is usually expressed in millimeters. In FORVM’s catalog descriptions you will usually find the coin’s weight, then maximum diameter, followed by the die axis.

We don't know the ancient denomination 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 Comparison

Since 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 Coins

U.S. Half dollar30.5 mm 2 euro25.75 mm
U.S. Quarter  24.1 mm 1 euro23.25 mm
U.S. Dime    17.9 mm 50 eurocent24.25 mm
U.S. Nickel 21.3 mm 20 eurocent22.25 mm
U.S. Penny   18.9 mm 10 eurocent19.75 mm
   5 eurocent21.75 mm
   2 Eurocent18.75 mm
   1 Eurocent16.25 mm

U.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

Inch and Millimeter Equivalents

Divide millimeter measurements by 25.4 to determine the inch measurement. 

Quick Fractional Reference Chart

Millimeters=Inch
3.17 mm =1/8 of an Inch
6.35 mm =1/4 of an Inch
9.52 mm =3/8 of an Inch
12.7 mm =1/2 of an Inch
15.87 mm =5/8 of an Inch
19.05 mm =3/4 of an Inch
22.22 mm =7/8 of an Inch
25.4 mm =1 Inch
31.75 mm =1 1/4 of an Inch
38.1 mm =1 1/2 of an Inch
44.5 mm =1 3/4 of an Inch
50.8 mm =2 Inches