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
C = 5/9 * (F - 32)
So GP's table is exact.
The stops are 33, 42, 51, 59, 68, 77, 86, 96, 103, which can also be read as ˚F.
The exact conversions are (˚C) 0.555, 5.555, 10.555, 15, 20, 25, 30, 35.555, 39.444 (I terminated the decimal repetitions, obviously)
The rule of thumb of 5˚ per stop is 0, 5, 10, 15, 20, 25, 30, 35, 40.
Note that eg. 51-59 is 8 blocks, 86-96 is 10 blocks, 96-103 is 7. But the rule of thumb is nevertheless good to ~half a degree ˚C the whole way.
C → F: times 2 then plus 30
F → C: minus 30 then divide by 2
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.
F = 2*(C-floor(C/10)) + 31
F = 2*C - round(2*C/10) + 32
F = round(9*C/5+32)
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...
- It is easy to remember that 0 °C = 32 °F because it is the freezing point of water.
- Every 10 °C interval corresponds to an interval of 18 °F. That's where the 9/5 in the conversion formula F = (9/5) * C + 32 comes from.
- Now it is easy to construct the lookup table: 10 °C = 50 °F, 20 °C = 68 °F, and 30 °C = 86 °F.
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.
In other words, (c - c/10) * 2 + 32 = 2c - 2c/10 + 32 = 9c/5 + 32.
The only difference between the refined approximation and your approximation is 31 vs. 32 in the last step. The rationale for choosing 31 is explained in the "Analysis" section of the blog post. To summarize, when we subtract 10% in the approximation method, we do not perform an exact division. Instead, we perform a floor division (discard the fractional part) for easier mental calculation. The floor division introduces an error that lies in the interval [0, 2). If we subtract 1 from the result, then the error lies in the interval [-1, 1). Therefore, in order to prevent the magnitude of error from exceeding 1 °F, we add 31 instead of 32 in the last step.
Also, I find subtracting 10% of smaller number from itself slightly easier than doing so with a larger number. That's why the subtraction step comes before the doubling step.
As an example of this weird mixture: today I drove 48 miles at 60 or 70 mph to walk 20km up a mountain that is 975m high because it is over 3000 ft high.
30 is hot 20 is nice 10 is cold 0 is ice
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 :)
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
30 C > 86 F
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.
Your comment about psychology is actually just you mistaking familiarity with your temperature system with something more universal. In countries where we use metric/Celsius, we find decimals super easy to think in because we’re used to doing it!
Everything you’re saying is all in your head.