I can't stand the fact that screws and nails are measured by the pound (at least in the US). What a ridiculous unit of measure to use for a count of something. I have never, not once, thought to myself "I need 2 pounds of nails for this job" because only a lunatic would think of "how many" in terms of weight.
While I agree with that when I need twenty nails, if I needed twenty million nails I would certainly want to buy them by the pound, instead of counting them one by one.
For very large numbers, it is always easier to count them by measuring either the total mass or the total volume of the items, instead of counting them, and then compute the numbers by dividing to the mass or volume of an individual item.
That is why in all chemical reactions the recipes are given by measuring the ingredients in grams or milliliters, even if what really matters are the numbers of molecules that must be combined. Because counting one by one the molecules is hopeless, the numbers of molecules are typically expressed as multiples of the Avogadro number, which is the number of molecules that would be contained in one gram of a substance whose molecular mass would be 1/12 of that of carbon 12.
So not only the nails, but also all chemical substances are usually counted by the gram, by the pound or by any other mass unit.
For large numbers, it would still be feasible to purchase them by number, and just employ a system to weigh them out, based on a knowledge of how many nails per Kg for a given variant (+/- a reasonable, small threshold).
Sure, and some things that are sold by number are measured like that. But when selling it can also create less disagreements to just skip to agreeing on what the scales say instead of having to agree on nails/kg.
If I need a handful of 5/16" screws, then odds are that I will also need nuts, washers and possibly lock washers. Instead of having to count out 10 each of all of that, I just grab a handful of all of them, toss them into a bag and the cashier weighs them and tells me the cost.
You could move to Europe, I've never seen screws and nails sold by anything except count, up to counts of 100-1000. Perhaps if I were to buy much more than that it would be by weight here.
you can buy trivial amounts (10-100) in small packages with precise count.
non-trivial amounts are sold by weight because no one is counting them. the approximate count per pound is usually specified.
I can't stand the fact that screws and nails are measured by the pound (at least in the US). What a ridiculous unit of measure to use for a count of something. I have never, not once, thought to myself "I need 2 pounds of nails for this job" because only a lunatic would think of "how many" in terms of weight.