I know many computer languages, but I’ve struggled all my life to learn a second human language. One of my problems is that I can’t stop trying to translate in my head. Just like Morse code, you need to understand things directly, not translate. But you have to start somewhere. One of the reasons metric never caught on in the United States is that it is hard to do exact translations while you are developing intuition about just how hot is 35 °C or how long 8 cm is.
If you travel, temperature is especially annoying. When the local news tells you the temperature is going to be 28, it is hard to do the math in your head to decide if you need a coat or shorts.
Ok, you are a math whiz. And you have a phone with a calculator and, probably, a voice assistant. So you can do the right math, which is (9/5) x °C + 32. But for those of us who can’t do that in our heads, there is an easier way.
Field Expedient
Most of us can’t multiply by 9/5 in our heads. But 9/5 is very nearly two. So if you double the Celsius temperature, you are halfway there. Of course, the number will be too high. But to make up for it, instead of adding 32, just add 30. For weather temperatures, this gives you a ballpark estimate. For 0 °C, you get 30 °F instead of 32. For 20 °C, you get 70 °F instead of 68. For 35 °C, you get 100 °F instead of 95. All close enough.
If you want to flip the error as the temperature goes up, you can remember to add 25 instead of 30 if the temperature is more than, say 25 °C. Then 35 °C gives you 95 °F on the dot, although other temperatures will still have some error, of course.
The error gets worse as the temperature rises, but it has to get fairly high before it gets useless. For example, my AMD CPU is currently at 48 °C. Using the +25 estimate, that’s 121 °F, instead of the correct 118. But maybe it won’t help you set up your metal smelting furnace.
Other Estimates
This is a useful way to embrace metric. Find rough estimates for units you deal with. For example, 2.54 cm/inch is not the easiest thing to apply. But if you remember that 5 cm is about 2 in, that works well. So a 160 mm rod is 16 cm. If you think of that as 3 x 5 + 1, you’ll know it is 6 inches plus an extra centimeter. The right answer is about 6.3 inches. Not close enough to start cutting things, but it does give you a feel for how big a thing you are talking about.
If you lived through the time when gasoline in the US went from less than $1/gallon to over, you might remember that many gas stations switched to liters because the pumps couldn’t be set for a dollar. The reason is a liter is very nearly a quart, and there are four quarts to a gallon. So 12 liters is practically 12 quarts or 3 gallons. This turns out to be very close.
Kilograms and kilometers are a bit trickier. The right way to imprecisely convert kilograms to pounds is to multiply by 2.2. But a nice mental math trick is to double it. Then remove the last digit and add the rest back in to the whole result. Then put the last digit you removed after the decimal point. So 8 kg would be 16+1 (throw away the six) or 17 pounds. Then put the 0.6 in for the correct answer of 17.6 pounds. Of course, the conversion factor isn’t exactly 2.2, but that’s what most people use anyway. If you are trying to be scientifically accurate, none of this is going to help you.
The factor for kilometers is roughly 0.6 miles in a kilometer or 1.6 km/mile. If you halve the kilometers, that will get you a fairly low estimate. So 35 km (21.7 miles) is easy to guess as more than 17.5 miles. That’s a pretty big difference, though. But if you then add 10% of the 35 back (3.5) you get 21 miles which is close.
Advice
I’m not trying to say that these rule-of-thumb tricks are good when you need an exact answer. But they are handy when you simply want a gut feel over some measure. Over time, you’ll just naturally know that 35 °C is summer-weather hot and you need more than a coffee mug to hold 3 liters.
Do you have a favorite fast conversion back or forth from metric? Share it in the comments. Americans love their strange measuring system. Turns out, some of the reasons we didn’t get metric was due to pirates, as you can see in the video below.
Featured image: Wood thermometer on white background by Marco Verch under Creative Commons 2.0
You are too optimist, most Americans aren’t even able to spell “Celsius” correctly (Selsius), let alone doing math from “Harry Potter” units to SI / IS :)
Sure, it’s spelled C E N T I G R A D E.
J/K
“aren’t even able to spell “Celsius” correctly (Selsius)” Challenge. Sounds like nonsense.
This article is for people who don’t know their times tables but still expects them to do multiplication.
All one needs is 0,1, and 2. 0 is ice, 22 is nice, 100? Not sure, stick your hand in it and let me know. I will also allow 3 because every ruler proclaims 30 cm per foot. And of course 6 for longer distances.
“The right way to imprecisely convert kilograms to pounds is to multiply by 2.2. But a nice mental math trick is to double it.” It’s adorable that you don’t notice that “the right way” is to do the “nice mental math trick”. Twice.
9/5 vs. 1.8: I must have been out the day we learned our 9/5 table, but multiplying by 1.8 is really just multiplying by .8 which is really just multiplying by 8. If 9/5 is easer for you have at it.
Anyone who follows this article’s advice just doesn’t value their time.
aniboti hoo ownli knoes wun wA 2 spel a wort ‘as knoh emadgeanation.
How do you spell Quadafee?
Arguing about letters used in foreign script words is a fools game…Fool.
Obligatory:
Imperial bolts have left the solar system!
How far have Napoleonic bolts gone?
We went to the moon, drove around and left cars (w Imperial bolts) behind.
We should have put them up on blocks before we left, marked our territory.
In their defense, one of them has a redneck bodged up fender made of cardboard.
Had been roosting moon dust like the General Lee.
“0.6 km/mile or 1.6 miles/km”
Are these units upside down (reciprocal)?
Being American, I had to make one mistake. The math is write, I just flopped the numbers in that sentence. Doh.
“I had to make one mistake. The math is write”
this is epically hilarious, thanks Al
I thought you might appreciate that.
From UK Kelloggs Cornflake boxes in the 1970s:
“Two and a quarter pounds of jam weighs about a kilogram”
“A litre of water’s about a pint and three quarters”. But UK pints and gallons are not the same as US pints and gallons :-D
Before car speedometers went digital in the UK they were marked in both mp/h and km/h. So easy to notice and remember that 50mp/h is 80 km/h.
I don’t see any point in “embracing metric” for household temperature measurments. Meters are great because all the units are powers of 10, and most scientific formulas are much cleaner. But for household use this does not matter at all. Now I get if you’re already using metric for everything else, why have a seperate scale for household temperatures, but in the US we’re already on a mixed system, so that doesn’t really matter either. Once we’re off of gallons, miles, inches and feet, then sure.
I’m actually not sure why we’re still so stuck on SAE. The old argument was that all the tooling is SAE, but now a huge amount of that is made in China and other metric countries anyway. Almost all modern machines support metric units. Just a matter of buying some new screws, bolts and drills.
“I get if you’re already using metric for everything else, why have a seperate scale for household temperatures”
It’s a silly argument anyway – metric already has multiple temperature scales, and the Kelvin/Celsius difference has caused problems with people who forget that temperature ratios only work with Kelvin or screw up the conversion.
And temperature itself is bull anyway – it’s not a real unit to begin with. In modern SI, it’s just a flat-out definition, it has nothing to do with water in any way shape or form anymore. 1 Kelvin is ~13.8 yoctojoules. Personally I prefer an indoor temperature of around 4.06 zeptojoules, but I understand people might prefer as high as 4.09 zJ. Anything under 4.03 zJ is just crazy, though.
People invent new scales all the time when it’s convenient. The scale doesn’t matter – the definition does, and Fahrenheit is defined based on SI units just like Celsius.
“I don’t know how hot or cold 80 deg F is outside” is a waaaay sillier than “how bad is 26 deg C”. Because with Fahrenheit, climate is basically “0 is too cold to be outside and 100 is too hot to be outside.” Interpolate between.
I definitely think of room and weather temperatures in F but for anything hotter than that which burns by touching it, it’s in C. It changes over around 140 F or so. I definitely have a “feel” for temperatures from about 180 C to 400 C (from 3D printing and soldering), but I have no idea what that is in F.
Come to think of it, that goes for other measurements too. Liquids smaller than 1 cup are in ml for me, anything larger than 2 liters is in gallons. Anything smaller than a pound is in grams. Anything longer than a few thousand feet is in miles, everything smaller than 2 feet is in mm, unless it involves feeler gauges, micrometers, manual milling machines, thin sheeting, or radial lathe dimensions, then it’s in thous. Axial lathe dimensions are sometimes in mm. Laser cutter, PCBs, 3D printing, and CNC milling are always mm.
“Most of us can’t multiply by 9/5 in our heads. But 9/5 is very nearly two.”
Yeah you can. 9/5 is 2*(1-0.1). So double it, and lop off 10%. Which is just sliding the decimal point back and subtracting. 24 deg C becomes 48-4.8 or about 43, add 32 and hey looky.
The scale difference between Fahrenheit and Celsius is 180/100, which is a useful conversion to know anyway.
Very similar to the 2.2 trick I mention later for kg. However, the double+30 is easier for non-math types and is quite good for temperatures normal people encounter.
Innumerate.
The word is ‘Innumerate’.
Like ‘Illiterate’ but for numbers.
Don’t use soft language to preserve the feelings of the dumb and lazy.
Unless you’re trying to F them, of course.
Should go without saying.
For livable temps, yeah. For cooking temperatures, less so. Using a temp of 390 F instead of 350-360 when a recipe calls for 180 C is kindof noticeable. Double it (360), lop off 10% (about 320-330), and add 30 (about 350-360).
You just have to know “you’re about 10% too high” and then it’s easy to think if it matters.
Reminds me of the approximate conversion tables in the first edition of Gamma World (the assumption being that the switch to Metric would be successful before any apocalyptic futures played out).
Although, then as now, there were many people too pedantic to see an estimate without having a panic attack. (“A meter is approximately a yard?! Egads!”)
As an American, I have a good feel for kilograms because of filament spools, and centimeters because a factor of 2.5 is not that difficult. On the other hand, my mental estimate for kilometers is about the same as my mental estimate for miles, which is to say, terrible
It’s ~5/8ths. Half it and add 10% of the original is a good estimate, within 5%. Even just “cut it in half” is only 20% off, which isn’t the end of the world.
100 kph/2 = 50 add 10% of the original = 60 mi/h, actual is ~62.5-ish. 300 kilometers = 150 + 30 = 180 miles.
Going the other way is actually easier (add half, you’re within 10%: add half and 10% of the original and you’re within 1%). 70 mph = 105 kph (not bad) -> add 10% of 70 -> 112 kph (within 1).
As an American who grew up in the ’70’s, I learned everything I needed to know from cartoons:
U.S. Office Of Education – “Metric Education” (PSA, 1978)
https://www.youtube.com/watch?v=JUXutV6Vk6k
As an european, I just sit back and enjoy the show…