In Australia nails are measured in mm for diameter and length, a common house nail is 100m long and 4.2mm in diameter. In the US, nails are measured in pennies. Wikipedia has quite a bit to say on this topic:
Penny sizes originally referred to the price for a hundred nails in England in the 15th century: the larger the nail, the higher the cost per hundred. The system remained in use in England into the 20th century, but is obsolete there today. The d is an abbreviation for denarius, a Roman coin similar to a penny; this was the abbreviation for a penny in the UK before decimalisation.
But this seems a little implausible, given that in the 15th century, wire nails were not actually available. In the talk section there is an alternative theory
In Folk-etymology: a dictionary of verbal corruptions or words perverted in form or meaning, by false derivation or mistaken analogy, published by G. Bell & Sons, 1882, Abram Smythe Palmer says, "Ten-penny nails are not nails ten of which may be got for a penny, but properly ten-pun'y or ten-pun'-nails, i.e., ten-pound, large nails, a thousand of which will weigh ten pounds (the old form of the verb to pound was pun). "It is surprising how slowly the commonest mechanical terms find their way into dictionaries professedly complete. I may mention, as instances of this, that penny, a denomination of the sizes of nails, as a six-penny or a ten-penny nail, though it was employed by Fently two hundred years ago, and has been in constant use ever since, is not to be found in Webster. — Marsh, The Eng. Language, p. 126 (ed. Smith). "Six-penny, eight-penny, ten-penny nails, are nails of such sizes, that a thousand will weigh six, eight, or ten pounds, and in this phrase, therefore, penny seems to be a corruption of pound." ...and he then goes on to quote a couple of usages from earlier literature (Abel Redivivus and Jokes and Wit of Douglas Jerrold). So, can anybody verify the origin of the term tenpenny nail as currently given in our article? rowley (talk) 20:08, 1 February 2012 (UTC)
This seems somewhat more plausible. Our methodology is to weigh 10 12D, 16D, 20D and 30D nails randomly selected from a paper bag of nails from the local hardware store. So let's see what the numbers look like:
the least squares line is y = (1.27*x/penny - 8.6) gram with 6.2 residual. So not a strong correlation, but still vaguely plausible. Perhaps the fact that these are hot dipped gal nails upsets things slightly? Anyway, so we take 1000m and it should weigh exactly 1 pound:
You have: 1.27*1000g You want: lb * 2.7998707
Well, close. I guess.