The way I convert is to remember that 32°F is 0°C and then count by "nines" for Fahrenheit to get the desired temperature in Celsius.

  32°F = 0°C
  41°F = 5°C
  50°F = 10°C
  59°F = 15°C
  68°F = 20°C
  77°F = 25°C
  86°F = 30°C
  95°F = 35°C

It's also pretty easy to remember that 10°C = 50°F and 35°C = 95°F if you want to go forward or back from there with the same counting method.

If you know the stops of the 6 train in Manhattan, the trick is that the stops are in Fahrenheit starting at 33rd street = 0˚ C. Each stop up town is +5˚C. Stays within 1˚C until 110th street.

A much faster approximation that is not too far off over the range of everyday non-extreme temperatures:

C → F: times 2 then plus 30

F → C: minus 30 then divide by 2

Personally I think the usual issue with the standard conversion is doing 9/5, but it actually isn't that bad. The way I convert is

C * 2 - C / 5 + 32, where I just round C to the nearest multiple of 5 before dividing to make it easy.

24C times 2 is 48, 24 / 5 ~= 5, so 43 + 32 = 75. Off by less than a half a degree, which (thinking about it for 15 seconds or so) I think is guaranteed under this system.

The most important thing is that this is easy to remember because it's basically just the real conversion, but also easy to do in your head.

The refined C to F conversion given is:

  F = 2*(C-floor(C/10)) + 31

Better is:

  F = 2*C - round(2*C/10) + 32

That's almost as easy to do in your head, but always gives the same result as doing the exact conversion and rounding that:

  F = round(9*C/5+32)

Also, if you would like the exact conversion, when you do the round(2*C/10) you can note how much the round changed the value, and that tells you how far off your final integer F temperature is from the actual value. You are high by that amount if the rounding was down, or low by that amount if the rounding went up.

Numerical implementations of functions often use look-up tables, and there's sometimes a trade-off between number of operations and memory access. (Large look-up tables aren't cache-friendly.)

This blog-post is from someone who prefers to do less mental arithmetic, but remember a few numbers: http://www.theexclusive.org/2012/08/converting-fahrenheit-in...

I've been avoiding doing mental calculations at all for this. I set my apple watch to display the temperature in Celsius, and every time I look at it, I get another data point in my mind to correlate to how it feels outside. This way, I can learn it independent from Fahrenheit.

Honestly I don't find this approximation any easier than the exact formula. They're each three steps. I was actually more excited to learn about the crude approximation that's only two steps, and the "memorize one conversion and go from there" method.

Edit: The original comment below did not take into proper account the significance of taking the floor in the approximations, rendering it mostly useless. ----

If the "refined approximation" assumes it's easy to 1) subtract 10% 2) double and 3) add 31, then it should be just as easy to 1) double 2) subtract 10% and 3) add 32 - i.e. the exact conversion formula.

I memorize some 10s. 0 is 32, 10 is 50, 20 is 68, 30 is 86. Extend beyond what’s memorized by using the ratio 10/18. I don’t have 40 memorized, but it’s easy to figure that it’s 104. Then use the approximation of 1C=2F to fill in the gaps. High today is 34? That’s 94. Thermostat says 72? That’s 22.

I'm very pro-metric, but Fahrenheit is nice for "human scale" temperatures. 0F is "very cold" and 100F is "very hot", with plenty of precision in between. 0C, on the other hand, is "kinda chilly" and 100C is "Oh my god, I'm dying".

It doesn't help much for people visiting here in the US, but when I travel outside the country I use this handy rhyme:

30 is hot 20 is nice 10 is cold 0 is ice

Does anyone have resources for other shortcut calculations for similar things? This particular one comes up a lot when speaking with my wife's family (they're from Mexico) but I've never tried to come up with a shortcut before. I'd love to see links to other resources, or recommendations for Math tricks for doing quicker mental math.

I remember the mnemonic "16 is 60". (as in 16C = ~60F)

Being a programmer I know 16*2 = 32 ~90. And its easy to remember that 0 is freezing... Covers most ranges of everyday conversation of temperature with non US friends :)

My heuristic goes like this

20C ~ 70F (ok it's 68F but close enough)

a variation of +10F is +5C (same for minus)

Of course you can remember that 32F = 0C, that helps

20 C > 68 F

30 C > 86 F

Or we can stop using freedom units and join the rest of the world. Take a small step to make this happen right now: set your phone and thermostat to show you the temperature in Celsius. I did this and my kids are growing up with an intuitive understanding of what 20 degrees C feels like.

For setting the a thermostat, it's not that hard to simply switch your brain over and know what's comfortable inherently in either F or C without doing a conversion back and forth. For accurate conversion, if c=(f-32)/1.8 or the converse is really that troublesome, Google can help you out.

I want to be careful, this could get holy war-ish...

So I do not want to sell anyone on changing their view. This isn't advocacy. But just want to provide one illustration of why someone may prefer Fahrenheit.

When I travel, hotel rooms generally let me alter the temperature on digital thermostats by one degree. In the US, that's great, that's plenty of precision. In Europe, I lose fidelity and am strictly worse off.

If people like a room set at 71 degrees, they don't like 72 or 70. If they like it at 75, they aren't secretly shooting for 76.

When I'm cooking in an oven or sous vide, I often want to tweak controls very precisely in an attempt to balance the carmelization of sugars or the rendering of fats while leaving proteins or starches intact.

Room temps, weather, and cooking are the ways I mainly interact with these scales. In each of them, the precision of the base unit in F is strictly advantageous to me.

Celsius can absolutely allow greater granularity, if everyone used an extra significant digit as a rule. I blame psychology though, people and systems often just don't bother to think that way.

I wholly support the metric system to unify measurement across different scales. That's neat. But as I rarely need to talk about millidegrees or gigadegrees, it seems less relevant to me in this context.

I don't find the range of temperatures of distilled water at sea level from freezing to boiling particularly useful for thinking about the weather. A more arbitrary scale like F works just as well maybe even better since 100F is too hot. However 0 as the temperature your seedlings will freeze is useful so C has that going for it.

You got them on meters and liters as well?

I did this years ago and it hasn’t worked yet.