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.
We also do not know the names of the denomination or value of many Greek and Roman provincial coins. Often the denomination given for these types 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, brass, base billon, or potin) coin with a 23 millimeter diameter.
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 dollar | 30.5 mm | 2 Euro | 25.75 mm | |
U.S. Quarter | 24.1 mm | 1 Euro | 23.25 mm | |
U.S. Dime | 17.9 mm | 50 Eurocent | 24.25 mm | |
U.S. Nickel | 21.3 mm | 20 Eurocent | 22.25 mm | |
U.S. Penny | 18.9 mm | 10 Eurocent | 19.75 mm | |
5 Eurocent | 21.75 mm | |||
2 Eurocent | 18.75 mm | |||
1 Eurocent | 16.25 mm | |||
U.K. Coins | ||||
2 Pounds | 28.4 mm | |||
1 Pound | 22.5 mm | |||
Fifty pence | 27.3 mm (7 sided) | |||
Twenty Pence | 21.4 mm (7 sided) | |||
Ten Pence | 24.5 mm | |||
Five pence | 18 mm | |||
Two pence | 26 mm | |||
One pence | 20 mm |
Divide millimeter lengths by 25.4 to determine the inch length.
MM | Approximate Size In Inches | Precise 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 |
Inches | MM |
.1 Inches | 2.54mm |
.15 Inches | 3.81mm |
.2 Inches | 5.08mm |
.25 Inches | 6.35mm |
.3 Inches | 7.62mm |
.35 Inches | 8.89mm |
.4 Inches | 10.16mm |
.45 Inches | 11.43mm |
.5 Inches | 12.70mm |
.55 Inches | 13.97mm |
.6 Inches | 15.24mm |
.65 Inches | 16.51mm |
.7 Inches | 17.78mm |
.75 Inches | 19.01mm |
.8 Inches | 20.32mm |
.85 Inches | 21.59mm |
.9 Inches | 22.86mm |
.95 Inches | 24.13mm |
1.0 Inches | 25.4mm |