It’s 18 degrees and lovely.

Didn’t take long — I found an excuse to include some math!

Oddly enough, people here speak of the temperature in Celsius (!).  This requires an American to mentally convert while conversing, which isn’t always easy, given that the conversion formula is \(F=\frac{9}{5}C+32\), not exactly convenient mental arithmetic.  If someone says it was 21 degrees the other day, you don’t really want to have to think through \(\frac{9}{5}\times 21^{\circ}\text{C}+32 = 37.8+32 = 69.8^{\circ}\text{F}\) before you can continue the conversation.

So I’ve taken to just doing \(F=2C+30\) instead, because 2 is close to \(\frac{9}{5}\) and 30 is close to 32.  Or the way I said it to the family: double and add 30.

But is that a bad shortcut or an acceptable shortcut?  Let’s compare my lazy formula to the real one:

(Blue is the lazy one, red is the correct conversion.)

Since you rarely hear about temperatures outside the \(0^{\circ}\)C to \(30^{\circ}\)C range, my lazy formula is off by at most \(2^{\circ}\)C with that conversion (or \(4^{\circ}\)F).  I find that acceptable.

To go from F to C is the reverse: subtract 30, then take half.  But we rarely need that, because people don’t say the temperature in Fahrenheit.  I switched my phone to Celsius for cultural immersion purposes.

Some folks in my family have complained that even \(2C+30\) is too much effort to make in the middle of a conversation, but as a mathematician, I’m professionally obligated to have no sympathy.

If you want to follow:


  1. ljc | | Reply

    This is helpful! And I also have no sympathy for the lazy family member(s) who can’t double and add 30.

  2. Owen | | Reply

    I found it easiest to remember this:

    0C = 32F, that’s cold.
    10C = 50F, that’s cool/chilly.
    20C = 68F, that’s warm.
    30C = 86F, that’s hot.
    40C = 104F, that’s really hot.

    So those five are easy to memorize, and then you can use the 1C ~ 2F to get something reasonably accurate from there. You’re never off by more than a degree F, an in air temperature you can’t feel the difference.

  3. Dana | | Reply

    Being the alternative is for the kids to force themselves to think in terms of Celsius, 2C + 30 doesn’t seem too bad to me either!

  4. Susan Trausch | | Reply

    Thank you Nathan (and Owen) for finally making Celsius sense to my math-impaired brain. Great to be traveling with the Carters. Especially love the Harry Potter landmarks. Know this will be a super year for you all.

  5. Jeff | | Reply

    I use the formula –

    5F = 9C + 160

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