### How to convert between Celsius and Fahrenheit in your head

When I lived in the Netherlands, along with my attempts to learn Dutch I also tried to get used to thinking of temperatures in Celsius instead of Fahrenheit. Of course, I’d grown up with the formula

$F = 1.8 C + 32$

for converting from Celsius to Fahrenheit, so in principle, anytime someone mentioned a temperature in Celsius I could work out what that temperature really was, though possibly needing pen and paper or an uncomfortably long pause in the conversation.

But that’s no way to live! Especially since my goal wasn’t just to understand what people were saying, but to learn to think the way they did, translating directly from “temperature in Celsius” to, say, “what clothing to wear” instead of via an annoying mental calculation. If I needed to convert a temperature, I wanted something approximate, not exact, so I could get as quickly as possibly to an interpretation of that temperature.

I ended up using two strategies that most seasoned Celsius/Fahrenheit converters probably already know about, but that I wouldn’t necessarily have thought of when I was first starting out. The first strategy just makes approximate conversion faster with a bit of memorization. The second strategy uses a little more memorization, but gets closer to the ideal of being able to interpret Celsius temperatures directly.

#### Strategy 1: Use known values to approximate nearby values

One of the easiest Fahrenheit/Celsius pairs to remember is that the freezing point of water (or, as we call it here in Minnesota, the “melting point”) is $32^\circ\text{F} = 0^\circ\text{C}$. If you know that, and you know that one degree change in Celsius is just about two degrees in Fahrenheit, that immediately tells you that $1^\circ\text{C} \approx 34^\circ\text{F}$, or $2^\circ\text{C} \approx 36^\circ\text{F}$, or $-1^\circ\text{C} \approx 30^\circ\text{F}$. If you can remember a few other starting points, you can do the same thing for most Celsius temperatures you encounter. Here’s a table of a few exact Celsius/Fahrenheit pairs:

 0˚C = 32˚F
10˚C = 50˚F
20˚C = 68˚F
30˚C = 86˚F

(Notice that 68 and 86 just have their digits reversed, so if you remember one you can remember the other.)

So if you want to know what $13^\circ\text{C}$ is in Fahrenheit, you can think, “That’s three degrees more than $10^\circ\text{C}$, so six degrees more than $50^\circ\text{F}$, or $56^\circ\text{F}$.” Or if you want to know what $85^\circ\text{F}$ is in Celsius, you can think, “That’s three degrees less than $86^\circ\text{F}$, or one-and-a-half degrees less than $30^\circ\text{C}$, so about $28.5^\circ\text{C}$.”

A couple other memorable starting places are

16˚C ≈ 61˚F
37˚C = 98.6˚F
-40˚C = -40˚F

That second one is actually the reason people think normal human body temperature is $98.6^\circ\text{F}$—in reality it fluctuates a degree or so over the course of a day, and the true average is actually more like $98.2^\circ\text{F}$, but at some point someone took an average of about $37^\circ\text{C}$ and translated it a little too exactly into Fahrenheit.

(And that third one is just there for the wow factor.)

I found this first strategy helpful for trying to cut down on time spent mentally translating temperature, but it didn’t solve the problem of not having a gut instinct about what a Celsius temperature actually meant.

#### Strategy 2: Convert temperature ranges, not temperatures

I realized that if I’m heading out the door and wondering whether I’ll need a jacket, I don’t need to know the exact temperature, just what range the temperature falls into. If it’s in the low sixties Fahrenheit, then yes, I’ll bring a jacket. If it’s closer to seventy, I probably don’t need one. It doesn’t really matter to me whether it’s 67 or 68—that’s more precision than I need. So I looked at a thermometer like the one below, and tried to see what some meaningful ranges of temperatures corresponded to:

Here’s what I came up with:

Low 0s C = 30s F
High 0s C = 40s F
Low 10s C = 50s F
Mid 10s C = about 60 F
High 10s C = mid 60s F
Low 20s C = about 70 F
Mid 20s C = high 70s F
High 20s C = low 80s F
Low 30s C = high 80s F
Mid 30s C = 90s F    

It took a bit of work to memorize, but then when someone said it was $12^\circ\text{C}$, I could think “mid fifties—a bit chilly!” without having to do any arithmetic at all. Or if I wanted to tell someone I didn’t want to go swimming unless it was in the eighties, I could say “I’d rather not swim until it gets into the high twenties.” That’s a lot closer to my goal of being able to think about Celsius temperatures directly, without having to convert at all!

Thanks for reading! How about you? Have you ever had to learn what a different system of units felt like? What helped you get used to it? I’d love to hear your story in the comments!

## 4 thoughts on “How to convert between Celsius and Fahrenheit in your head”

1. GIULIO says:

Hi Owen! Nice post. What about using
2*C+30
for an approximate Farenheit temperature? The error will not exceed 4 Farenheit in the range of temperatures suited for human life 😉

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1. Thanks, Giulio! That’s exactly correct for 10C = 50F, which is a nice middle. Unfortunately, a corollary of your formula is that Minnesota summers and winters are both regularly unsuitable for human life!

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2. David Sprunger says:

I faced the same problem moving to Japan, though I think any American moving anywhere will have this issue! 🙂 One nice fact I noticed for easing mental calculations is that 1.8c + 32 = 2c – 0.1(2c) + 32. You can make various approximations from there, like 0.1(2c) ≈ tens digit of 2c, 0.1(2c) ≈ 0 (like Giulio suggests) or 32 ≈ 30, depending on how accurate you want to be.

But I think the biggest help was switching the weather app on my phone to display temperatures in C and just slowly absorbing what various temperatures felt like.

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1. That’s a really nice way to think about it! I did the same with the weather app on my computer, and got pretty good with the range of temperatures that were common in Leiden.

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