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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

Inch and Millimeter Equivalents

Divide millimeter lengths by 25.4 to determine the inch length.

MM
Approximate Size In Inches
Exact Size In Inches
1mm
1/32 Inch
0.03937 Inches
2mm
1/16 Inch
0.07874 Inches
3mm
3/32 Inch
0.11811 Inches
4mm
1/8 Inch
0.15748 Inches
5mm
3/16 Inch
0.19685 Inches
6mm
Almost 1/4 Inch
0.23622 Inches
7mm
Little over 1/4 Inch
0.27559 Inches
8mm
5/16 Inch
0.31496 Inches
9mm
Almost 3/8 Inch
0.35433 Inches
10mm
Little over 3/8 Inch
0.39370 Inches
11mm
7/16 Inch
0.43307 Inches
12mm
Almost 1/2 Inch
0.47244 Inches
13mm
Little over 1/2 Inch
0.51181 Inches
14mm
9/16 Inch
0.55118 Inches
15mm
Almost 5/8 Inch
0.59055 Inches
16mm
5/8 Inch
0.62992 Inches
17mm
Almost 11/16 Inch
0.66929 Inches
18mm
Almost 3/4 Inch
0.70866 Inches
19mm
Little over 3/4 Inch
0.74803 Inches
20mm
Almost 13/16 Inch
0.78740 Inches
21mm
Little over 13/16 Inch
0.82677 Inches
22mm
Almost 7/8 Inch
0.86614 Inches
23mm
Little over 7/8 Inch
0.90551 Inches
24mm
15/16 Inch
0.94488 Inches
25mm
1 Inch
0.98425 Inches



- 101

Ancient| |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





Diameter 101

The 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. 

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 lengths by 25.4 to determine the inch length.

MM
Approximate Size In Inches
Exact Size In Inches
1mm
1/32 Inch
0.03937 Inches
2mm
1/16 Inch
0.07874 Inches
3mm
3/32 Inch
0.11811 Inches
4mm
1/8 Inch
0.15748 Inches
5mm
3/16 Inch
0.19685 Inches
6mm
Almost 1/4 Inch
0.23622 Inches
7mm
Little over 1/4 Inch
0.27559 Inches
8mm
5/16 Inch
0.31496 Inches
9mm
Almost 3/8 Inch
0.35433 Inches
10mm
Little over 3/8 Inch
0.39370 Inches
11mm
7/16 Inch
0.43307 Inches
12mm
Almost 1/2 Inch
0.47244 Inches
13mm
Little over 1/2 Inch
0.51181 Inches
14mm
9/16 Inch
0.55118 Inches
15mm
Almost 5/8 Inch
0.59055 Inches
16mm
5/8 Inch
0.62992 Inches
17mm
Almost 11/16 Inch
0.66929 Inches
18mm
Almost 3/4 Inch
0.70866 Inches
19mm
Little over 3/4 Inch
0.74803 Inches
20mm
Almost 13/16 Inch
0.78740 Inches
21mm
Little over 13/16 Inch
0.82677 Inches
22mm
Almost 7/8 Inch
0.86614 Inches
23mm
Little over 7/8 Inch
0.90551 Inches
24mm
15/16 Inch
0.94488 Inches
25mm
1 Inch
0.98425 Inches



- 101

Ancient| |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