The Letter ‘d’ in Nail Sizes

Updated: Apr. 03, 2023

The shocking truth about nail sizes! What they don't want you to know

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Carpenters always talk about 16d common nails or 8d finish nails or 10d sinkers, but nobody ever explains what the heck ‘d’ means. This article does.

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Understanding ā€œdā€ sizes

Nails

For historical reasons, nails are sold both by a number followed by d and (less confusingly) by length.

The ā€œdā€ stands for penny, so 8d refers to an 8-penny nail, 16d to a 16-penny nail and so on. Itā€™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 ā€œdā€ stand for penny? The ā€œdā€ goes way back to the time when the Romans occupied what is now England. The ā€œdā€ is associated with a Roman coin called the denarius, which was also the name for an English penny.

So whatā€™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 ā€œdā€ indicate that one 16d (3-1/2 inch) hand-forged nail cost 16 pennies.

Who knows which version is true, or why the ā€œdā€ 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.

Size Conversion Chart

Print out this handy reference chart and tape it in your tool box, just in case you forget.