I'm not stupid, I'm just American!
November 16, 2012 6:00 PM   Subscribe

I need to learn how to seamlessly convert between miles and kilometers in my head - without having to think about it.

I'm in the US - where we're raised on the English measurement system, not metrics. Just about the only metrics I've ever been exposed to is knowing that a 10k race is 6.2 miles long.

I've been in aviation for years, and I can dance pretty easily between mph, knots, nautical miles, etc... but now I find myself at work regularly in meetings and presentations where everything is in kilometers.

I find it pretty much impossible to sit there and instantly grasp just how far a distance, speed, etc is being presented. I try to convert it with simple math in my head, but just can't do it fast enough to keep up with the discussion and still have a real understanding of the material.

Can anyone provide tips on how you've learned to accept metrics without having to jump through mental hurdles? If someone tells me they were traveling at 70 mph, I inherently 'know' what that is like, or if they travelled 500 miles I can look at a map and make an accurate guestimate of how that looks. But kilometers? Or meters per second? I'm lost.

Is there a simple way to make the conversion without having to actually, you know, MAKE the conversion? Is there a technique where I can just 'get it'?
posted by matty to Education (25 answers total) 11 users marked this as a favorite
Best answer: You could begin by memorizing basic conversions like 10, 50, 100, 500 km into miles. Then if someone says "150 km," you'll just KNOW what 100 km and 50 km 'feel' like and can add them together. It'll be a start.
posted by showbiz_liz at 6:04 PM on November 16, 2012 [2 favorites]

Best answer: I learned it by consciously paying more attention to the KMPH markings on my car's speedometer. 60mph = 100(ish) 30=50. Once you start paying attention to that, it starts coming more naturally.
posted by deadmessenger at 6:06 PM on November 16, 2012 [2 favorites]

Can you divide it in half quickly enough? Not exact, but close enough for most purposes...

For a bit better (and actually pretty damned close), divide it in half then add 10%. Both those operations, most people can do almost instantly, so...
posted by pla at 6:08 PM on November 16, 2012 [1 favorite]

Division by 2 and 10 is very easy for most folks. So I suggest that when someone uses kilometers, you just divide that number by 2 and add the same number divided by 10. 20km is roughly 10mi + 2mi. 56km is roughly 28mi + 5.6mi.
posted by Monsieur Caution at 6:08 PM on November 16, 2012 [14 favorites]

Best answer: And when you find the system that works for you, practice, practice, practice. I got a job that required me to think in 24 hour time (1600 instead of 4pm, for example) and I had to stop and consciously think "add 12 hours to that if it's PM, so that will make it 1600".

Once I started using 24 hour time in my personal life (meeting so-and-so for coffee at 4pm, right, that's 1600) just for practice, it soon became automatic. I can use both together and not get confused.
posted by malibustacey9999 at 6:19 PM on November 16, 2012 [1 favorite]

Using the knowledge that 10k is 6.2 miles you could derive a few basic conversions from kilometers to miles, and vice versa.

If 10K = 6.2 miles, half of 10K would be half of 6.2 miles.

Therefore, 5K = 3.1 miles.

Likewise, multiply everything by 10 and then you have 100K = 62 miles.

1000K = 620 miles
1K = .62 miles

Now, if you want to remember the conversion from miles to kilometers you could force yourself to remember just one equation and then derive everything from that one equation. So, if 10 miles = 16 kilometers, then,

1 mile = 1.6 K
100 miles = 160 K
1000 miles = 1600 K
posted by nikkorizz at 6:25 PM on November 16, 2012

Pla and Monsieur Caution use the same system I do (I'm Canadian and learned km). And yeah, practice. When you're driving along, and you see the posted signs, say it out loud.

Also: do you actually need meters/second?
posted by Lemurrhea at 6:26 PM on November 16, 2012

Best answer: Incidentally, this question caused me to learn a new way to convert km to mi and vice versa. Since the ratio of miles to kilometers is approximately equal to ϕ, you can shift values in the Fibonacci sequence to get an answer.

Not sure that's helpful, but probably the best way to 'get it' is to have fun with it by observing stuff like that.

(PS. to pla: LOL, no worries. :D)
posted by Monsieur Caution at 6:29 PM on November 16, 2012 [6 favorites]

Roughly speaking, five miles is eight km. If you can multiply things by eight (elementary school times tables), it is a snap. 300 miles is 6 times 50 miles, and 6 times 80 is 480, so 300 mi = 480 km. In fact, 300 miles is 482.8 km, so close enough.
posted by ricochet biscuit at 6:29 PM on November 16, 2012

When driving on roads with kph markings (eg: 100 kph), multiply times 6 and drop a zero to get a rough mph.

100 kph x 6 equals 600, drop a zero gives you roughly 60 mph
posted by blueberry at 6:59 PM on November 16, 2012

For mental conversions, I use 0.6 and 1.6, which are close enough. One kilometer is about 0.6 miles. One mile is about 1.6 km. I'm thinking it makes sense to make a list of distances or speeds you can picture in your mind. Empire State Building = 443 m. Mount Everest = 8,800 m. A marathon = 42.2 km. DC to NYC = 380 km. East-west distance across Texas = 1,200 km. NYC to Los Angeles = 3,960 km. Earth to Moon = 380,000 km. Earth to Sun = 150,000,000 km. Speed limit in a school zone = 30 km/h (20 mph). Highway speed limit is 100 km/h (60 mph). Typical walking speed is 5 km/h (3 mph). Human running speed record is 45 km/h (28 mph).
posted by Dixon Ticonderoga at 7:07 PM on November 16, 2012

Switching between miles per hour and meters per second is actually pretty easy, because they are roughly a factor of two different. More precisely 1 m/s = 2.24 mph. So for getting a quick feel for what the speeds mean, take the m/s number and double it to get mph.
posted by medusa at 7:22 PM on November 16, 2012

Best answer: Oh man, I was of the age in Canada where it switched mid-elementary school to Metric, so I feel your pain. I still do imperial height and weight, but everything else metric.

So First I can't resist: Bob and Doug McKenzie explain how to convert to metric beers] Sadly, "double it and add thirty doesn't actually work" except for Celsius to Fahrenheit, sort of.

I like the 1 km = .62 miles method myself, but in truth every car I've used has both metric and imperial on the speedometer. I times it by .5 (50%) then 10%, and add them together.
posted by chapps at 8:09 PM on November 16, 2012

Best answer: I think it's helpful to get some touch points in your head. Like, "my daily commute is 12 km." "I can jog at 8kph." "Driving on streets is about 60kph." (This summer, I switched my weather app to metric and learned that 45C feels like death.)
posted by hishtafel at 9:02 PM on November 16, 2012 [2 favorites]

Best answer: Find out things like your height, weight, daily commute, car's weight, speed, computer screen size... things which you directly experience. Instead of thinking "that's 5 miles" think "that's a bit less than the 9km school run". He weighs 100kg? That's a lot more than me/he's my size
posted by Wrinkled Stumpskin at 10:18 PM on November 16, 2012 [1 favorite]

Monseiur Caution has a good technique, and similar to the one I use for lbs to kg and back.

Also, you can use the Fibonacci sequence as an approximation.

3 miles is about 5 km.
5 miles is about 8 km.
8 miles is about 13 km.
13 miles is about 21 km.
posted by the man of twists and turns at 11:13 PM on November 16, 2012

This is maybe a 'times table' kind of problem. One does not develop skill at rapidly adding 7+7+7+7+7+7; one just memorizes 7x6=42, 7x6=42, 7x6=42. If you drill it enough, your brain hardwires it and the answer is instant.

How to drill -- maybe get Excel to generate a whole pile of random numbers, and then convert each of them from miles to km (or whatever), even just using a calculator. Do them one at a time, so that you have to write or type the converted answer, which gives your brain time to notice what's going on. After enough practice you will be able to guess the answer instantly, I'm certain.
posted by PercussivePaul at 11:30 PM on November 16, 2012

It depends how accurate you need to be - whether you're actually calculating or just want a ball park idea as the numbers flash by in conversation. I'm in the UK, and do a bit of running, and flip between miles and km in my head a lot, but I don't need to be particularly accurate. For the most common distances I just know the approximate conversion (10k is roughly 6 miles). But apart from that I just use the rule of thumb that the distance in miles is about two thirds of the distance in km, and on occasions I need to be accurate, convert it on Google.
posted by penguin pie at 12:07 AM on November 17, 2012

for approximation I use:
miles * 1.5 = km
km * 2/3 = miles
posted by DarkForest at 4:33 AM on November 17, 2012

Best answer: Peeve. Grrrr.... A mile is 5280 feet of 12 inches each? Why? What idiot did that?

All great ideas above, but it's just measurement. Start with a map of your locale. Mark off some distances for reference. Get really good at multiplying by .6 and 1.6. That is 90% of it, and you are in a land where no one will contest your results. If you are off by 10%, it's irrelevant. Your mileage estimates are wrong, too.

Until the frame in your head is adjusted, it'll be uncomfortable, but it's like learning French. Vocabularies and structure are additive, not exclusive, right?

Precision does not matter unless it matters, as in machining parts or making microcircuits or building a house. Even in the case of the latter, it doesn't matter what the size of the measuring stick it, as long as there's only one. For distance measurement, points of origin and destination are gross anyway.
posted by FauxScot at 4:54 AM on November 17, 2012

Response by poster: Thanks everyone for your input! I'm already putting the tips into practice - especially the speedometer on the car, as my truck has kph beneath the mph. Much appreciated!
posted by matty at 7:53 AM on November 17, 2012

To do this kind of stuff in your head, you have to break it down. Miles is km * 1.6. KM is miles * .62. So miles is km * (2 - .4) and km is miles * (.5 + .12)

The Fibonacci sequence is great for middling values. Obviously, it's wrong that 1 mile is 2km, or 3 km is 2 miles. But after that, it's plenty accurate. Until you get to values where it's hard to remember the sequence any more.

To me, this seems easiest:

miles to km: (miles * 2) - (miles/10*4)

So 1000 miles is 2000 - 400 = 1600 km. You are only off by 9km, or half a percent.

Or 341 miles is 682 - (34 * 4). Which is 682 - 120 - 16 = 546 Or 682 - 32 - 32 - 32 - 32 = 546. Whatever is easiest to do in your head.

You can also divide and conquer- 341 is 300 + 40 + 1, so you do:

300mi = 600 - 120 = 480
40mi = 80 - 16 = 64
1mi = 2 - .4 = 1.6
= 545.6

Going in the other direction is (km/2) + (km/10) + (km/100*2)

1000 km is 500 + 100 + 20 = 620

Dividing and conquering, 497 km can be rounded to 500, and then it's 250 + 50 + 10 = 310. To correct your rounding, you can then do 3km = 1.5 + .15 + .06 = 1.71, round it and subtract it. 497 km = 308 miles.

If you skip the km/100*2 part, you are still only 2% off.
posted by gjc at 7:55 AM on November 17, 2012

Peeve. Grrrr.... A mile is 5280 feet of 12 inches each? Why? What idiot did that?

Surveyors. A mile is 8 furlongs, and a furlong is 10 chains, and the chain was the physical tool that surveyors took out to the field to make measurements and record property boundaries. It was made up of 100 standard size links, for a total of 66 feet in length.

Also, 10 acres is 10 chains^2.
posted by hwyengr at 9:56 AM on November 17, 2012 [1 favorite]

Just double it then subtract a few. For greater distances, subtract more.
posted by lalala1234 at 10:50 AM on November 17, 2012

"The Metric system is the tool of the devil! My car gets 40 rods to the hogshead and that's the way I like it!" - Grampa Simpson

Also, a nanocentury is approximately pi seconds.
posted by blob at 7:24 AM on November 18, 2012

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