These are conversions I know off the top of my head, I didn't need to look them up. Which is the point the GP was making: it's not hard to memorize the handful of conversions you will encounter in everyday life. Most people living here did it as children and have never had to think about it since. That's why there's no actual gain for us to switch to metric units. On the other hand there would be quite a bit of pain as everyone had to adjust to estimating things in kilos vs pounds, grams vs cups (in recipes), and so on. So for the typical American, it is actually a net negative for the country to switch to the metric system. It isn't just stubbornness.
What's clumsy about 30cm though? If you are working at scales where this level of precision is needed, you can just use cm throughout, and the beauty of metric is that even someone who has never had to do that before will know immediately how much it is because conversion to meters (or millimeters, or whatever the primary unit is in their usual applications) is so easy.
Similarly, I've heard similar sentiments expressed about lack of pound equivalent in metric. But in practice we just say "500 grams" etc (and for bonus points you get 400 grams, 300 grams etc).
Miles and yards are both used as units of distance, so conversion is obviously relevant. The only reason why "yards in a mile" doesn't come up all the time is because Americans work around it by subconsciously (?) avoiding any such cases where the conversion is non-trivial. E.g. a road sign in Europe might say "400 m", whereas in US a similar one will be "1/4 miles".
And "evolved from hundreds years of usage" generally means a lack of internal consistency, because most units originated a long time ago as a way to measure something very specific - in many cases, something completely irrelevant to most people using those units today. Nor did those units remain consistent through history - just look at how many definitions ounce has in US in different contexts, all of them historical! Or regular vs nautical vs survey mile. Even just cleaning up that mess would be a massive improvement.
This is not true and that's trivially verifiable.
No calculator, no references, no Google:
How many inches are in 5 and three-quarter miles?
How much does 5 & 3/4 gallons of water weigh?
The point is that it is easy to interconvert units in the metric system: they're all interrelated and all of them can be expressed in terms of other units.
A litre is the volume of a cube that is 10x10x10cm. A cubic cm of water weighs a gram. Therefore, 10 cubic cm of water weighs 1kg. In other words, a kilo is the weight of a litre of water.
It takes 1 calorie of energy to heat 1g of water by 1ºC. So to heat a litre by one degree takes 1000 calories, or 1 kilocalorie. To boil it, you need to raise its temperature to 100 degrees, because that's the boiling point of water. From room temperature of say 20º that means
100-20 = 80 80 x 1000 = 80,000 80 kcal
This is useful. You can work out how much energy you need, using a pencil and paper.
This makes it easy to convert. Units that are easier to convert are more useful.
Whereas, as Josh Bazell put it:
« Whereas in the American system, the answer to ‘How much energy does it take to boil a room-temperature gallon of water?’ is ‘Go fuck yourself,’ because you can’t directly relate any of those quantities. »
That's stupid. They can easily be related, but the people who devised this batshit set of scaled random numbers a few centuries ago didn't think of it.
It's not "what you're used to is better".
It's "one system is easier to work with than the other, and lets you do things the other system does not permit you to do."
Metric is objectively, demonstrably better than imperial.
And the common response of "oh but my system is what people feel" is also total bullshit, because that is 100% what you're used to.
A warmish day is feels like 20º to me. A hot day feels like 30º. A burning can't think straight day is 40º. Below freezing is below zero: it's simple and intuitive.
Except they didn't actually, see my points about the UK (similar points apply to Canada).
