The “d” stands for penny, so 8d refers to an 8-penny nail, 16d to a 16-penny nail and so on. It’s a way to indicate nail length, as you can see in the table below.
Now for the obvious question: Where does the sizing system come from and why does “d” stand for penny? The “d” goes way back to the time when the Romans occupied what is now England. The “d” is associated with a Roman coin called the denarius, which was also the name for an English penny.
So what’s that got to do with nails? It was associated with nails when they were hand-forged, one at a time. Some say a hundred 3-1/2 inch nails would have cost 16 pennies and thus became known as 16-penny nails. Others say that the number and the “d” indicate that one 16d (3-1/2 inch) hand-forged nail cost 16 pennies.
Who knows which version is true, or why the “d” designation persists even in the building codes today, but fortunately, retail boxes of nails are marked with the penny size as well as the length in inches. As you can see in the photo, length is only one factor of many in choosing a nail.