In my experience, Europeans from metric countries find units like gallons and inches mystifying and the effort required for them to learn them is hard to justify unless they regularly visit or do business with the USA or UK. So when speaking their languages, I use metric measurements unless I am talking generalities e.g. I walked for miles. In my lifetime, the UK has in fact transformed into a metric country for almost all professional and legal purposes, with a few everyday exceptions like miles, mph, and mpg for travel; and pints for milk and beer. Of course aviation worldwide uses feet for altitude.

"I'll give you the US CUs, but the differences are small. And I wasn't meaning that anyone can just mathematically convert units, but roughly compare them in everyday situations. For example, an 8oz bottle of water compared to a 1L bottle of pop. That was my error for not being specific." Well I'm not sure which way you are going. But, no, the differences between U.S. Customary and Imperial are not small. In Orwell's novel, "Nineteen Eighty Four," that's why the old guy at the bar is complaining bitterly that the bartender will only draw him a liter or a half-liter, when he wants a pint. A half-liter is only 80% of an Imperial pint. (I couldn't figure that one out as a kid because I "knew" that a half-liter is almost identical to our "pint" and is actually just a bit bigger, so why would anyone complain about getting a half-liter instead of a pint?) Talking to a U.S. speaker it depends entirely on whom you are talking to and what their experience is. In the special case of a liter, it just happens (for reasons I don't understand) that 1 liter is a very common soda bottle size, so many of us know that one liter is about a quart. We have converted our athletic fields to Olympic standards but many of us are still very hazy on how long a kilometer is, or how fast 100 kph is. I think they are now using kilometers in the military, which they call "klicks." Despite spending many years earning a degree in zoology and theoretically knowing SI, I never know how hot or cold the weather is, expressed in °C, without converting. Until recently I wouldn't have known whether a man 180 cm. tall was very tall or very short. I know my weight in pounds but can't tell you even roughly in kilograms without actually doing some kind of calculation. Most of us use and think in U.S. Customary most of the time.

We _don't_ use imperial units, we use U.S. Customary units. In Ye Olde Days before Canada went metric, Canadian "gallons" and "quarts" were 25% larger than U.S. "gallons" and "quarts." Probably fluid ounces and drams too, although I don't know. And in the U.S. people are not generally familiar with the "metric system," unless they are scientists or medical people--and even then, they use metric units for certain things and don't convert mentally. A friend of mine whose an HVAC engineer knows heat in BTUs and pressure in PSI and can't convert them mentally to SI. I don't know what people tolerate; my Latin American friends seem to know the Spanish words like "mila" (mile), "pie" (foot), and "pulgada" (inch; I always forget and say "pulga" which means "flea.") However it is clear that they have no idea whatsoever about temperatures in °F. I don't know what the etiquette is but I always convert, to me it is part of "translation," and to me learning SI is just part of learning Spanish. It's not so much etiquette is that, like you, I just can't be sure what my listeners will or won't understand. Since Google will do conversions I can even do it in the middle of a conversation, it only takes a few seconds. And I keep a little picture of an old-style analog thermometer with C and F scales, taped up next to my screen where I can glance at it during a conversation.

Brooke, your initial statement is wrong. Americans do not use both Imperial and Metric. What did you read that made you think that? Every other country in the world except America uses Metric (with a few exceptions in the UK), so they won't tolerate Imperial units.

They're not 'roughly interchangeable' A meter and a liter. But that's about it. 0C is not even close to our Fahrenheit of 32 I'm confused as to why you'd believe this. You're in the U.S., right? Are you a native AmE speaker? Your grammar doesn't read like you are.