In the United States, it depends. International units are not weird, but we do not understand them instantly. We don't "think metric," we have to convert them, and we may or may not be able to do it mentally. From experiences like watching the Olympics, driving in Canada, and participating or watching "5K" and "10K" races, most of us know the words "meter" and "kilometer" and have a good idea of how long they are. Military people use kilometers and call them "klicks." In high schools science classes we see "meter sticks" which look just like "yardsticks" and know that a meter is about a yard (3 feet). We also know from soda bottles (!) that "one liter" and "one quart" are about the same. (Warning! That's a U.S. quart.) If you want to be sure of being understood and you are talking or writing specifically for a U.S. audience, yes, you should convert lengths to inches, feet, or miles; weights to pounds; volumes to fluid ounces, quarts, or gallons; land area to acres or square miles; and temperature to degrees Fahrenheit. A very convenient way for this is to use Google! A Google search on "40 km to miles" will not only give the answer but brings up a little units converter! If you are addressing an international audience, just use international (SI) units and let us U.S. readers worry about it. Scientists and doctors use the metric system in their work and understand it. By the way: speaking as a U.S. citizen, shame on us. We should be using the international system. In the 1970s were well on our way to converting. For a while, road signs on the Interstate highways showed distances in both miles and kilometers. The political climate changed and the conversion effort was abandoned.

In the UK, it isn't a problem at all. We use both systems.

Yes, you may confuse people from the USA, one of two countries in the world that have not yet subscribed to the metric system. I think people of the UK would be fine with it.

Americans use feet, the rest of the world uses metric system. You will not confuse anyone because people know how to adjust. The internet makes everything possible ^^

Looks like I still have a lot of things to learn.