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Comment by slashink | Hacker News Reader

Yes, first thing i thought about. I sadly never learn to have a simple conversion baseline between these two, even if i travel a lot between the different continents.

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One pound is half a kilogram, three feet is a meter[1], one inch is the size of my thumb. No, they're not precise, but it's easy enough to memorize, close enough to understand a text. (I also use one mile=2km but I have the notion the error on that one is huge, although I never remember if it is actually 1.8km or 1.6km)

For Fahrenheit, I never managed to memorize a conversion rule. Weatherwise 0 is friggin' cold, 100 is friggin' hot seems to be enough for daily consumption.

[1] Actually, my mnemonic is that I measure 6ft (1.82m), but I don't think my mnemonic is useful for anyone else.

For a relatively accurate guess to go from miles->kilometres you can use the fibonacci series. If the current number is in miles, the next one will be the same distance in kilometres. Works from 2 onwards.

So from 2 3 5 8 13 21 ... you can see that 3 miles is 5 kilometres, 8 miles is 13 kilometres etc.

Fahrenheit is pretty easy. The key temps are 30, 60, 90:

  -30 is dangerous, hibernate.
    0 is frigid, wear a thick coat and warm pants.  
   30 is cold, wear a coat. (freezing point of water)
   60 is cool, wear clothes.
   90 is hot, no clothes.
  120 is roasting, God help you. (melting point of humans)

Bonus Temps:

  72 is room temp.
  78 is time to wear shorts.
  48 is time to wear shorts in cold regions*.

* That is, in any place where people don't understand why finding outlets to plug in electric cars is a problem. Sure, you can't currently plug them in at the store, but your garage has power. What's that you say? Well, of course many people live in apartments and don't have garages, but they can just use the outlets in the parking lot. What, you don't have outlets in your parking lot? Well where do you plug in your gas powered car? You know, for the heater? To warm it up enough to start when it's 30 below? What tropical island do you live on?

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