When describing nail sizes, Americans use an English penny
designation from the 15th Century.
A
10-penny nail never cost 10 pennies.
In current usage, a 2d nail is 1" (25mm) long. Each 1d increase is
a ¼" (6mm) increase in length up to 10d, and a ½" (12mm) increase
after that.
One explanation of the origin of the penny as a nail size is that
the size of a nail in pennies was the price in pennies of 100 such
nails.
Another theory says that the pound was once abbreviated "d". The
"d" size of a nail was the weight in pounds of 1000 nails; that
is, a thousand 2d nails would weigh 2 pounds. Confusion between
"d" for penny and "d" for pound did the rest.
The chart below shows the "imperial" and "metric" equivalents.
 |
|
Penny |
Inch |
Millimetre |
|
2d |
1 |
25 |
|
3d |
11/4 |
30 |
|
4d |
11/2 |
40 |
|
6d |
2 |
50 |
|
8d |
21/2 |
65 |
|
10d |
3 |
75 |
|
16d |
31/2 |
90 |
|
20d |
4 |
100 |
|
40d |
5 |
125 |
|
60d |
6 |
150 |
|
|
