"If someone is an adult, it's a bunch of new information to learn, and unless they work in a field that involves measurements and math, it probably won't be an obvious net positive for them."
Absolutely true! So what does a country do to overcome the problems of familiarity and habit? First thing is not to scare the population and the best way to do that is with a friendly and sophisticated advertising campaign.
Before I go further I must point out that Australia, the UK and New Zealand had a much more difficult task than the US if or when it converts to Metric. Reason: our currency followed the LSD system—Pounds, Shillings and Pence—so we had the double-sized problem of converting both the currency and weights and measures. Right, the US has had decimal currency almost from its beginning. You've already a head start! :-)
Pre decimal currency people in Australia, the UK and NZ had a mad system inherited from history where 12 pence = 1 shilling, 20 shilling = 1 pound (£). And there was an even madder unit called the guinea (gn) which is 21 shillings—that's 1 shilling more than the pound or 252 pence/pennies (that's madness but there are good historical reasons for it that go back centuries).
It used to be commonplace to see ads, store sales etc. like 14gn & 3/- (shillings)—that's 297 shillings or 3,528 pennies (if I haven't screwed the math up).
And that was only part of it, there's a florin, a crown, half crown etc and a halfpenny. Everyone had to know all this stuff (if you didn't you'd likely be robbed). A couple of examples in your parlance:
penny (1d) — 1¢, penny
sixpence (6d) — 5¢, nickel
shilling (12d) — 10¢, dime
10 shillings (10/-) — $1, dollar
Every country that converted from LSD to decimal adopted the preferred 1-2-5 number series to minimize the number of coins, i..e.: 1, 2, 5, 10, 20,…. Same with the Euro. The US has somewhat screwed its coinage up with its beloved Quarter. When those of us who come from 1-2-5 series countries go to the US one of the first things we notice is how much loose change we accumulate in our pockets. The difference is amazing!
https://en.m.wikipedia.org/wiki/Preferred_number#1-2-5_serie... https://en.m.wikipedia.org/wiki/Coin_problem
It's worth noting that Australia's main currency unit is the dollar (same basic structure as the US dollar) which is half the old pound. That was too much for the UK, tradition held them back, the old 20 shilling pound became the new 10 shilling pound sans name change. That meant a coarser granularity, the smallest unit (the new pence) is over twice that of the old. Of course, that led to price hikes. The UK then went on screw up its weights and measures conversion for the same reason. I'll discuss why in a moment.
As mentioned, everything counts on getting the population on side, here's one of Australia's decimal currency ads (even now it's pretty good):
https://m.youtube.com/watch?v=Qm_Vtl2u1Hc †
A similar campaign was waged a little later with the weights and measures conversion. This is where Australia shined and it suggests your worry about conversion being a tough sell—which is true—has to be accepted and tackled head on. And that's how it was done here.
What Australia's Metric Conversion Board did was to cold-turkey the whole country from the outset just as happened with decimal currency—old coins and notes disappeared almost overnight, as soon as 'old' cash was banked new coins and notes were issued.
Almost immediately, grocery items changed from pounds and ounces to kilograms/grams, pints/fluid ounces to litres, feet and inches to metres/cms. Suddenly, kids going back to school after holidays could only buy 30cm rules sans inches—both sides were only metric.
What's more the government made it unlawful to sell new goods with Imperial measurements on them. Right, supermarket goods didn't have both pounds/ounces and kilograms—only kilograms (of course manufacturers had time to tool up).
What happened? Well, a lot of whingeing for a few weeks and it was essentially all over. People adapted remarkably quickly. Teachers got rid of all those Imperial tables, and so on. Lumber changed, a 4x2" (OK, 2x4" in your lingo) became 100x50mm, and so on.
Oh, and I must say I lived though all that, I was a teenager when decimal currency came in. Even I bitched about not being able to buy a rule with both inches and centimeters on it. In fact, I had a friend who worked for the Metric Conversion Board and I used to whinge to him about it. It also worked both ways, I always had the latest lowdown on how the conversion was proceeding.
What happened after that? The country adjusted remarkably well and it now thinks metric by default (and that's absolutely the key point). Both the US and the UK are largely metric in areas but neither think in metric by default. It's not a cultural meme in either.
After everybody in Australia had become fully metrified the government reversed the ban on things marked with Imperial measurements (you'd never see groceries marked in Imperial units but you would see dual units on a 30cm rule nowadays. It was a great success with little pain.
Now look how the UK stuffed it up—or what not to do! The UK started its metrication program in 1965 which is earlier than Australia yet it's still a mess. Officially it's a metric country and yet a huge amount of Imperial activity is still taking place. I recall around the time of Brexit some of Boris Johnson's cohorts were even calling for the reintroduction of Imperial units. Moreover, you see the Imperial mindset everywhere even in the BBC, for example, doctors on medical programs frequently discuss patients weigh in stones—for heaven's sake very few people in Australia would know what a 'stone' is let alone that it's 14lb.
What the British did was do precious little, they didn't ban goods marked with Imperial measurements and so on, 60 years later it's still largely an Imperial country.
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Edit, Ha, I've not seen that clip for many years but looking at it now I realize that it was produced by Artransa Park TV which was a subsidiary of the television network I worked for. I'd probably would never have made that connection if it were not for this post. Small world eh?
