Lighten up everyone, arguing about metric versus imperial, and the luxury of being certain of which to use - you should try being British and of a certain age...
We were taught metric at school but everyone outside of the school seemed to work in Imperial. Helping your Dad became a nightmare. The results were never impressive and involved learning a lot of new swear words.
I guess the results can be called the 'British Dad' system, It goes something like this...
You measure things in mm, cm and metres. However, when you run out of tape measure you switch to feet and yards to estimate further. Miles are used if it's really big distance, especially if you don't want to go there. (Kilometres are reserved for school text book problems and the army.) Temperature is easy - use metric in the winter ("Blimey - I'm not going out there it's 5 below") and Imperial in summer ("It's a scorcher - 80 in the shade!"). Weights - in the gym your expected to use kilos, in conversations I tend to use stone (whatever they are, scales in our house when we were kids were always marked in stone... Estimating how many bags of sugar someone is - is very useful, that give you an estimate in kilos ). Liquids - use ml for small stuff and pints after that. Mind you if it's more than a couple of pints switching back to litres helps... ..
It's not two competing systems, it's three. The USA uses their own american system of measurements, based on, but not quite the same, as imperial. Biggest difference is is the Pint, american pint is 20% less than an imperial one. Some of us need to learn all three systems because a few can't be bothered to use the world standard.
Canadian system is similar to "British Dad" system... we "officially" converted in the 1970s (iirc) but because of a certain, um,
stubborn neighbour with a dominant culture and manufacturing segment the transition has not been total. Roads are marked in Km, but when talk about how far away a place is Canadians will tell you how long it takes. "How far is it to Torrana?" "'Bout an hour, eh? If you stop for a Timmy's it'll add about 15 minutes."
Our weather reports are in Celsius, and in our house the outdoor thermometer is also Celsius ... but the thermostats for the heat are still Fahrenheit. Ironically, I can change the the thermostats to read Cº, but that doesn't make sense to me. I'd rather tell my wife I'm cranking the heat to 72º since they're calling for -6º tonight (1st in F, the 2nd in C).
Our beer comes in ml, if it's in a can or bottle, but by the pint if you're in a bar. Whether it's a British pint or the 20% smaller American pint is anybody's guess.
I've seen a sign in a supermarket "Buy a 2lb ham, and get 250g of potato salad free!" When you pay for the ham, it's rung up in kilos of course.
We ask for a gallon of paint, but we get 3.78l. I don't know if this is the British or 20% smaller US gallon. But if you don't need a gallon you ask for litre, and get 946ml.
If you are shopping for milk you ask for a gallon, and get 4 litres. Or you ask for a litre and get a litre. But if you want half a litre you ask for a pint, which is 500ml.
I'll drive several km to buy wood in feet and inches. We'll measure the size of our house in sq ft, but tell you how big the lot is in metres.
This is all based on my generation's experiences - I was still in highschool during the height of the transition. Our younger nieces and nephews think we're crazy, and are mostly metric.
As a photographer working in a darkroom, I took to metric volume measurements very early. With the "Imperial" system, you never really knew if you were working with the American or British sized units. Was the measuring cup made in the USA calibrated for the American market or the Canadian market? Metric was far far easier.