Centigrade to Fahrenheit conversion. Simple. Really.

[note: I'm not satisfied with this, yet and will continue revising. There's something simpler that I'm missing, maybe just in the explanation. I'd LOVE!!!! to get feedback from somebody willing to sit down for 15 minutes and try this stuff out. You'll invest your time well, and maybe figure out what I'm missing.]

The almost-simple way to mentally convert Centigrade to Fahrenheit is to double the number, subtract 1/10, then add 32.
Ex. 5ºC = 5*2=10.
10 - 10/10 = 9.
9+32=41ºF.

Sometimes, though, if a conversion doesn't come quickly, the brain just stops and moves on to other pressing issues. This is the dreaded plotz. Rather than risk that, it's worth learning how to get a reasonable estimate quickly and then, if needed, an accurate conversion.

For quick and simple:
Double the number and add 30.

This gets us really close enough for most situations that aren't conducive to picking up a pencil. It only requires two steps.

To convert 5ºC to Fahrenheit:

5*2=10.
10+30 = 40ºF.

How accurate is it?

Anywhere between freezing (0ºC, 32ºF) and shirt-sleeves weather (28ºC, 68ºF) this rough result is within two degrees, being 2º low at freezing, 2º high at shirtsleeves, and absolutely correct in the middle (10ºC, 50ºF).

Getting away from 50ºF in either direction, the rough answer creeps more and more off. At 0ºF and at 100ºF, it's 5 degrees off.

So, if a closer figure is needed, two additional steps makes it accurate to the nearest whole degree.

Remember, 50ºF is the center.

If that rough result Fº is higher than 55ºF, it needs to be pulled back in one degree. If the rough result was more than as 65ºF then two degrees back. If it was more than 75, subtract three. If more than 85 subtract 4, etc.

To let the procedural part of the brain handle this (not the plotz-prone intellect), use fingers. Count the tens from 50 up to the rough estimate (but only count the last ten if the final digit is > 5). Pull the estimate back down by as many degrees as counted on your fingers.

23ºC= 23*2+30 gives a rough result of 73, which is more than 55, so it has to be pulled back down.

Starting at 50, count one. 60 is two fingers. 70 does not count, since 3 (in 73) is not more than 5 .
73ºF (rough estimate) -2(fingers) becomes 71ºF (accurate)

On the colder end of the scale, if the rough result is as less than 45, extra degrees have to be added to pull it back in toward the center. It's a finger count of the tens going down, starting with 40. Count the final ten ONLY if the final digit is < 5.

Example:

3ºC= 3*2+30 gives a result of 36, which is lower than 45.

Starting with 40, we finger-count one , but we do not count the 30, since the last digit of 36 is not <5.

36ºF (rought estimate) -2(fingers)= 34ºF (accurate)

(If there's trouble remembering whether the final digit needs to be greater or less than 5 to be counted, think if it as being closer or further away from the center (50ºF). For warm temperatures, count if it's >5, for cold, count if it's <5.)

With a little practice, this should be easy to do. Much easier than explaining it.

For about 5 minutes, practice getting a rough estimate. Take each number below, double it and add 30.
2ºC 1ºC 10ºC 4ºC 6ºC 13ºC 20ºC 5ºC 9ºC 11ºC 33ºC 3ºC 15ºC 25ºC 49ºC 35ºC 40ºC (start over if it's not been 5 minutes)

For 5 more minutes, practice converting a rough Fahrenheit estimate to an accurate conversion.
50ºF 66ºF 78ºF 96ºF 106ºF 92ºF 62ºF 104ºF 58ºF 70ºF 48ºF 52ºF 44ºF 30ºF 34ºF 38ºF 46ºF 78ºF 88ºF 66ºF 42ºF 102ºF

For 5 more minutes, do a rough estimate, then bring it in to accuracy.
2ºC 1ºC 10ºC 4ºC 6ºC 13ºC 20ºC 5ºC 9ºC 11ºC 33ºC 3ºC 15ºC 25ºC 49ºC 35ºC 40ºC 10ºC 11ºC 8ºC 20ºC 22ºC 11ºC 25ºC 30ºC 12ºC 5ºC 9ºC 2ºC 8ºC 1ºC 0ºC -2ºC

For optimum retention, do all this again later in the day.

After a little practice converting Cº to Fº will be easy, but to make sure it sticks in habit, practice twice a day for two more days, then at least once more later in the week. This can be practiced while driving by taking any numbers in sight.

Part of the strategy here is that in order to multitask as pilots are always expected to do, the intellect determines the need for some cognitive work, but can assign that work to other faculties,freeing the intellect until the result comes back for more evaluation. Cool, huh?

The spreadsheet version of this information is here.