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Understanding “d” sizes
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
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.
Size Conversion Chart
Print out this handy reference chart and tape it in your tool box, just in case you forget.