6 ms·
Temperature Conversion: Mental Calculation (2014)
- imurray 7y agoNumerical 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-into-celsius.html http://www.theexclusive.org/2012/08/converting-fahrenheit-in...
- susam 7y agoThanks for sharing the theexclusive.org link. The trick discussed there is quite nifty. Here's a nice way to recall the lookup table mentioned there, in case, one forgets it: - 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.
- jiananli_ 7y agoA 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
- thedanbob 7y agoThis is what I use. I'm not quick with arithmetic in my head so if I want anything more accurate it's faster to just grab my phone.
- thisacctforreal 7y agoFWIW iOS Search (swipe down from the middle of the Home screen) can do conversions too. https://i.imgur.com/16AAqDG.png https://i.imgur.com/16AAqDG.png
- bscphil 7y agoBoth of these are in the article, under "crude approximation". The criticism made in the article was that it can be off by ~3 degrees Fahrenheit at normal temperatures.
- philipps 7y ago20 C > 68 F 30 C > 86 F
- technothrasher 7y agoFor 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.
- saagarjha 7y agoFor setting a thermostat, it rarely matters enough for you to know the exact value you want. I can get within a degree or so just by memorizing a handful of conversions and then interpolating between them.
- raverbashing 7y agoMy 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
- evanb 7y agoIf 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.
- gowld 7y agoSo, every 9 streets from 33rd?
- evanb 7y agoNot quite. 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.
- bscphil 7y agoPersonally 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.
- Ragib_Zaman 7y agoEdit: 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.
- susam 7y agoThat's right. In fact, the latter would be just as easy as 1) subtract 10% 2) double 3) and add 32, i.e., the exact conversion formula again. 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.
- Ragib_Zaman 7y agoAhh fair enough. I should have spent more time understanding your rationale. I'll edit my original comment to reflect that.
- IgorPartola 7y agoOr 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.
- duckymcduckface 7y agoYou got them on meters and liters as well?
- IgorPartola 7y agoLiters don’t come up much and those are going to be a lot harder given that every gas station is in gallons only. Meters no, centimeters yes. They are 6 so they mostly measure small stuff. I use grams for cooking too. Who thought that “1 cup of cheese” was a reasonable measurement?
- kortilla 7y ago>Who thought that “1 cup of cheese” was a reasonable measurement?, It’s easier to use a 1 cup scoop to grab something quickly than it is to weigh something out.
- rat9988 7y agoWhich cup?
- IgorPartola 7y agoIn case you are not from the US, an official freedom unit of volume is a cup which equals 8 fluid ounces (not the same as ounces which are a unit of weight; yeah it really is that dumb). Almost all cooking recipes use teaspoons (5ml), tablespoons (15 ml), or fractions of a cup (1 fluid oz, 29 ml or so), including for measuring things like cheese, herbs, pasta, etc. No American recipe website will ever say “use 200 grams of butter), it will tell you how many cups of butter to use.
- rdiddly 7y agoHonestly 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.
- Davertron 7y agoDoes 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_complete_me 7y agoNo resource but there is a similar conversion trick for kilometers to miles. Divide by two and add a tenth. viz: 60 km = 60/2 + 60/10 = 30+6 = 36 miles. Not exact but near enough for many cases.
- michelpp 7y agoIt 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
- mikeash 7y agoI 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.
- hyperpape 7y agoIf you are ok with being slightly off, you can combined memorizing tens with the approximation that 1C = 2F. Going up or down from the nearest 10C, you’ll never be off by more than 1F.
- u801e 7y agoThe 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.
- tomger 7y agoFor weather purposes the approximation seems accurate enough. Here’s a table listing the differences: http://tomgermeau.com/tools/FC/ http://tomgermeau.com/tools/FC/
- bonyt 7y agoI'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.
- softwaredoug 7y agoI 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 :)
- tzs 7y agoThe 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.
- JshWright 7y agoI'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".
- arethuza 7y agoI'm not convinced - even in the UK which has a mixture of mostly metric and some Imperial units in common usage all temperatures are in C and I've never heard anyone complaining about it. 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.
- IgorPartola 7y ago0 in C is useful as the point of freezing. Very useful for determining things like whether rain is likely to turn to ice on roads, etc. I guarantee you that if you grew up using Kelvin, you’d be defending it right now. Spend a year looking at nothing but Celsius and you won’t need to go back: 0 and below is cold, 10 is spring time, 15 is jacket optional, 20-25 is warm, and 25+ is hot. That’s all you really need.
- JshWright 7y agoI'm not "defending" it... I'd be all for a wholesale switch to metric units. I simply said it's "nice" in one specific context.
- IgorPartola 7y agoAh that’s fair. Sorry I misread that.