It's only the US and its associated territories , plus the Bahamas, Belize and the Cayman Islands that officially use Fahrenheit. Everywhere else uses Celsius so this might become a rather repetitive thread!
What happens in the UK is rather strange. We use Celsius, but when it gets hot here people are just as likely to say it's 90 degrees (Fahrenheit) as it's 25 degrees (Celsius.) But for colder temperatures, we would always say it in Celsius.
When we visited some friends in the Netherlands, we were wandering around a local public park that looked pleasantly old-fashioned, and on a post there was a rather old-looking thermometer that read the temperature in °R, degrees Réaumur!
I was very excited because when I studied high-school physics in the late 1950s our textbook mentioned three temperature scales: Fahrenheit (freezing = 32°F, boiling in 212°F), "Centigrade" (as the Celsius scale was always called then) (freezing = 0°C, boiling = 100°C) and Réaumur (freezing = 0°R, boiling = 80°R). I had never, ever, ever seen a Réaumur thermometer or a Réaumur temperature before. Our Dutch friends, by the way, had never really noticed the thermometer, and had no idea what the Réaumur scale was.
It's said that the seemingly-crazy Fahrenheit scale was intended to put 0°F at the lowest temperature easily reached in a laboratory with ice and salt, and that 100°F was intended to be body temperature. I have no idea why Réaumur set 80° as the boiling point... well, according to Wikipedia, https://en.wikipedia.org/wiki/Réaumur_scale , it was basically a confused mess...
In the United States we use the Fahrenheit scale and even those of us fairly familiar with the international system find it hard to "think" in degrees Celsius. I know that freezing is 0°C, room temperature is 20°C, and body temperature is 37°C and I use these as very rough landmarks. And I keep a little C-to-F conversion chart next to my computer screen to glance at when I'm talking to language companions.
By the way, your mention of 40° bring up another trivia item. The very cold temperature of -40°F is also exactly -40°C. In fact, I find it easier to convert between the two scales by adding forty, multiplying by 5/9 or 9/5, and subtracting forty, than to remember what to do with the number 32.
Australia adopted the metric system in 1974. The changeover went smoothly and a couple of generations have grown up using the system which is common to most countries. You will still hear people describing a person's height in feet and inches as well as centimetres but everything else is expressed in metric units including temperature.
Ironically, the country which still sticks in large part to the traditional British system (with some local variation) is the country which first threw off the shackles of empire, the United States.
By the way, when talking of temperature, it is still surprising how many people in the northern hemisphere, don't understand that the seasons are reversed in the southern hemisphere. Christmas in Australianand most of South America and Southern Africa is a summer festival.
Indeed, Paul. It's very sad because in the 1970s it appeared that the United States (my native country) was finally about to convert to the "metric system" (as we still call it). Road signs on the Interstate highways system began to give distances in both miles and kilometers, car speedometers had dual scales, and I still have a glass measuring cup in our kitchen which has both fluid ounces and milliliters marked on it. On television, meteorologists always announced the temperatures in both Fahrenheit and Celsius.
Metric conversion became a strange casualty of the Reagan era. I'm not entirely sure of the reasons behind it.
Of course, all of the U.S. Customary units have long been officially defined in terms of their metric equivalent--the legal definition of "one inch" is 25.4 mm.
I worked for years at a company that built high-tech gear, and it was very odd because everything was actually built to inch dimensions but all of the drawings showed metric dimensions--everything was 2.54 cm or 25.40 cm or 101.60 cm in length. Everything in medicine is metric, of course. When I was a child in the 1950s you still saw bottles of aspirin tables labeled "5 grains" instead of "325 mg," but the grains and minims and drams are long gone. And, thanks to the influence of the Olympics, most races and tracks are now in SI units. (When I was a kid, a standard running track was 440 yards long, and I remember my pleasure at realizing that that was, in fact, two furlongs, and that a mile (5,280 feet) was eight furlongs.
We have already lost at least one expensive spacecraft, an unmanned Mars probe, that crashed because of a misunderstanding as to which system was being used.
Here's a related question, for English speakers. How do you pronounce the word "centigrade?" The majority of US speakers pronounce the "cent-" in "centigrade" the same way as the words "cent" or "sent."
However... and this is particularly true of people in the medical profession... a small number of people pronounce it as "sahntigade," as if paying respect to the French origin of the word. (It's like the similar variation in the pronunciation of the words "envelope," "envoy," and "enclave.")