For many Americans, the Centigrade temperature scale lies somewhere between foreign/unknown and communist/un-American. However, there are many benefits to becoming “bilingual” in your temperature scales, including being able to understand international travelers as well as fit in better when traveling internationally, to being prepared for the inevitable transition from Fahrenheit to Celsius like many other countries have done in living memory.
In school, you likely learned the mathematical formula for converting between the two scales. The two main problems with this method are: (1) many people are bad at mental math, so multiplying or dividing by 1.8 or 9/5 is nearly impossible, and (2) it does not transition a person into fully understanding and feeling the new scale. When learning a new language, your goal is not to simply translate what you want to say in your native language into the newer language. Your goal is to say—as naturally as possible—what you want to say in the new language. So learning Celsius must be done in the same way.
First, you need to learn the framework of Celsius. Once you get a handle on that, then you will be better at natively expressing temperatures in C.
Where to begin? I start everyone off with what I call major points on the F to C scale. These lay the framework that can be filled in with more detail later. In school, you start with 0 C equals 32 F. However this is too cold—too far to the left on the scale—to be a good starting point. I start everyone off with a more common and easy-to-remember conversion. 50 F is 10 C. That’s easy enough to remember; they both end in 0.
From there, we go warmer and cooler to the other common major points. I’ll list them out for now and explain the math later for those who are interested.
10C = 50F
20C = 68F
30C = 86F
40C = 104F
Now let’s go cooler by a couple:
0C = 32F
-10C = 16F
-20C = -4F
That’s both warm and cold enough to cover what most people experience on a daily basis.
Now for the math behind the major points. We started at 10/50. From there we add 10C to get to 20C. For the F, we need to add 18F. So 20C (10C+10C) = 68F (50F+18F). We repeat this math two more times. 30C (20C+10C) = 86F (68F+18F). 40C (30C+10C) = 104F (86F+18F). 40/104 is also easy to remember due to the 0s and 4s.
The math is opposite and equal in the colder direction 50-18=32 and 10-10=0, so 0C = 32F, and so on.
I suggest you simply memorize these major points. There are only a handful, and memorizing them will make more precise conversions go faster later.
Another note on C. You can think of the major points as clothing changers. 0 is cold. 10 is sweater weather. 20 is nice but slightly cool. 30 is warm. 40 is hot.
Now for the minor points. Similar to the major points, we will start at 10/50 and step up and down. This time we will only move by 5C and 9F each time. So 15C is 59F (50F+9F). The next minor point is a previous major point (20/68) so we can skip it. One more would be 25C = 77F (68F+9F). You can tell the minor points end with 5C. 35C = 95F (86F+9F). This one is also easy because 35=95. Both end in 5. And if you want, 45C =113F (104F+9F). Very hot indeed.
Going down, 5C (10C – 5C) = 41F (50F-9F). We already did 0C (32F). -5C = 23F (32F-9F). And -15C = 7F (16F-9F).
At this point, you have the framework for 80% of the Celsius scale that you would encounter on an average day. If you can memorize the major points and memorize how to get to the minor points, you will be very far ahead of most Americans.
A couple of quick exercises: 72F. We have two options to start from here: we know 68F is a major point at 20C. That’s close. We also know 77F is 25C. Also close. They are both 5 F from those points. So we can pick one and call it a close day or split the difference and be really precise. Halfway between 20C and 25C is 22.5C, so let’s just say 72F is 23C (if we are rounding).
89F. This one is quicker. We know 86F is a major point in 30C. We could stop that and be close. Or increase it by one or two and be more precise. Let’s say 89F is 32C. It might not be exact, but we are close enough to know if we need to wear shorts or pants.
Now on to more precise conversions. Once we have the framework of major and minor points, we are no more than three C away from one of those points (because the minor points are only 5C apart from each other). So now all we need is a way to get between those. Let’s say we are given 69F. We know 68F is 20C exactly. So we are only 1F away from that exact major point. For each degree C, we increase or decrease F by 1.8. Don’t worry: we will only have to do this twice at most. So 21C is exactly 69.8F (68F + 1.8F). Nos, we are past 69. So is 69 closer to 68 or 69.8? Clearly, it is closer to 69.8, so we can say, with rounding, 69F is 21C.
If it were further away, we would add 3.6F and 2C. So 22C is exactly 72.6F.
We never need to go past 2C and 3.6 F because that would put us closer to the next major/minor point, at which time it would be smarter and faster to go from that point down or up.