_________________________________________
    |                  |    Approximate     |
    |                  |   Number per lb.   |
    | Penny  | Length  | Common | Finishing |
    | Number |(inches) |  Nails |   Nails   |
    |   2d   |  1      |   875  |    1350   |
    |   3d   |  1 1/4  |   575  |     900   | 
    |   4d   |  1 1/2  |   300  |     610   | 
    |   5d   |  1 3/4  |   300  |     500   | 
    |   6d   |  2      |   175  |     300   | 
    |   8d   |  2 1/2  |   100  |     200   | 
    |  10d   |  3      |    70  |     125   | 
    |  12d   |  3 1/4  |    65  |     115   | 
    |  16d   |  3 1/2  |    50  |      90   | 
    |  20d   |  4      |    30  |      75   | 
    |  30d   |  4 1/2  |    20  |      -    | 

Not to be confused with a pennyweight.

The d symbol is a traditional abbreviation for a penny, deriving from the Latin denarius. There is a very rough correlation between the length of a nail and its nailweight: an n-penny nail is about .5 + (.25n) inches long. As you can see from the above table, even that approximation skews off on the longer nails.

The origins of this measurement have been lost in the mists of time. The common guess is that you could get a hundred n-penny nails for n pennies in Merry Olde England.