Medicine For People!

January 2018: Decomplexifying Our World - The Metric System

Road Sign Miles Kilometers

Decomplexifying Our World: The Metric System

Why should you concern yourself with learning more about the metric system? It's complicated and you don't need it, correct?

Well, think. Our planet is no longer the vast and inexhaustible wilderness it was a few hundred years ago. As the human population has ballooned to 7.6 billion, the numbers of animals we raise for our food far outweighs the numbers of remaining wild animals.[i] We are rapidly turning our former richly populated wilderness into agricultural land, significantly reducing the variety and amount of plant life on the planet even as we are consuming an ever larger proportion of that plant life for our own sustenance.

To compound the problem, we are poisoning the atmosphere.

Fellow humans, we are in a fix.

As a physician, I see people struggle to grapple with the sometimes complex concepts they must understand in order to make intelligent decisions about their own health and how to maintain it. Making intelligent health decisions includes decisions about our environment, so necessary to our very existence. I enjoy making complex ideas easy to understand as part of the guidance I offer my patients.

I know that many Americans have trouble with the metric system used by pretty much all the rest of the world. Today I offer you some tricks that will allow you to easily assimilate it into your own understanding.

Airport Security and the Metric System

As I was returning from my holiday trip to see family, the TSA officer reminded us travelers that liquids were not allowed in quantities of more than 3.4 ounces. 3.4 ounces? Seemed like an odd number, until I realized that 3.4 ounces is very close to 100 milliliters. Got me wondering if one day our country will finally realize we can learn from other nations and complete the transition to the metric system.

Is it better? Well, it's certainly simpler.

Actually, we're halfway there already. Look at your food labels: grams and calories—both are metric units. Your car? Metric bolts, screws, everything. Your earbuds have 2.5 millimeter (mm) plugs; all your DVDs are 120 mm. Science classes? Metric. Medical school? Metric since at least 1965.

How hard would it be for all of us to switch to metric, really? Having used both metric and English measures for most of my working life, I offer you this quick cheat sheet.

The Main Units: Not That Different

A liter is pretty much the same as a quart. Unless you are doing chemistry or buying and selling carload quantities of liquids, the difference isn't going to matter. Since a liter is close to a quart, half a liter (500 milliliters, abbreviated ml) is close to a pint.

The meter? Pretty close to a yard. Can you see much difference in length between a 100-meter-long soccer field and a 100-yard-long football field? Neither can I.

Other units are not so equivalent, but it's easy to convert from one to another.

In medicine we commonly chop the meter up into 1000 parts called millimeters (mm) or 100 parts called centimeters (cm). Cent, get it, as in parts of a dollar? The width of your little fingernail is close to a cm, the length of the little finger from the joint to the tip is about 25 mm (an inch, in English measurements, or 2.5 cm).

If you've ever raced the Turkey Trot, you know that 5 kilometers is about 3 miles. A kilometer is roughly 2/3 of a mile. It's 20 miles to the nearest city? Call it 30 km and that's close enough. That rule of thirds works in both directions. You see a Canadian road sign saying 90 km to Edmonton? Since miles are longer than kilometers, there are going to be fewer miles. I use 2/3 as my rough estimate and mentally call it something close to 60 miles. Actually it's 55.9 miles, but either way you'll spend about the same amount of time behind the wheel.

The degree Celsius (or centigrade) used by our neighbors to the north and south? Roughly equivalent to 2 degrees Fahrenheit.

The kilogram? A little more than two pounds.

Technically, of course, the exact equivalences are not quite so precise. You have to multiply a kilogram by 2.2 to get pounds, but if you are after enough potatoes for that salad, how close do you have to get? You want to guess how far that ship is out in Admiralty Inlet, you're gonna be about as accurate guessing in meters as you are going to be in yards. Those are the major elements of your day-to-day metric system. Forget about converting volts, ohms, hertz and a whole host of other scientific measurement units. You're already using metric.

Let's Get Technical

Geek alert! Yes, if you're in a science class or working in a technical field, you're going to have to do math for exact English to metric conversions.

For those times when you need quick numbers, here are rules of thumb for when that cell-phone isn't handy.


Divide a liter by a thousand to get the milliliter (ml). Five ml is a standard teaspoon, 15 ml is a tablespoon, 30 ml roughly an ounce.

Since a liter (1000 ml) can occupy a cube that's 10 cm on a side, a thousand liters is 100 cm on a side, therefore a cubic meter, so just over a cubic yard. That's a lot easier than wondering how big a container you'd need for 250 gallons. Figure that your gallons is 1000 quarts, therefore close to 1000 liters, therefore close to a cubic meter and a cubic yard. You can look at any fuel tank now and guess the capacity!


To convert degrees Celsius to degrees Fahrenheit, just remember that 0 degrees C = 32 degrees F. Take the Celsius degrees (say 10 degrees), double that (20), subtract 10% (20-2=18), and add to 32 (32+18= 50). So when it's 50 degrees F outside, it's 10 degrees C.

Now when you hear the weather report in Vancouver B.C., you can quickly tell that 0 degrees is overcoat weather, 10 degrees may require a jacket, 20 degrees is the ideal temperature, and at 30 degrees be sure to wear shorts or plan on a shower when you get home.


Metric makes going from volume to weight easier, too. A liter of water weighs a kilogram, or about 2.2 pounds. This means that a half-liter is about 1 pound, which is why the Brits like to say "a pint's a pound the world around."

Back to that 3.4 ounces. An ounce is a little less than 30 ml. So three ounces is a little less than 90 ml, and, as I said in the beginning, 3.4 ounces is close to 100 milliliters. A nice round number, much easier to remember, isn't it?

There are times you'll want to use a calculator for these conversions, but most of the time when you wonder about metric/English conversions, these rules of thumb will be all you need.

Our Complicated and Distressing World

Just like the crew of the starship Enterprise, we are more and more dependent upon each other and our self-contained island universe, our planet Earth. Like that crew, we need to pay attention and not let complex ideas flummox us.

So this little bit of complexity, English/metric conversions, is a great place to start. Don't be afraid. Just remember a few of these rules for today, and add on as time goes by!


[i]Sapiens by Yuval Harari quotes information from MIT Press in 2002. Harvesting the Biosphere: What We Have Taken from Nature by Vaclav Smil from MIT Press in 2012, quoted by Bill Gates at Smil estimates human aggregate weight at 125 million metric tons, domestic animals at 300 million, total 425 million metric tons. Surviving wild land animals (from the size of a mouse on up), 10 million tons on the entire planet. If you weigh 170 pounds, there is, on your behalf, a total of 4 pounds of wild animal left on the planet.

Alternatively, consider a 2018 world population estimate of 7.6 billion, with 26% being children between the ages of zero and 14 years, and take an average child's weight of 60 pounds and an average adult weight of 140 pounds, you end up with an estimate of about 450 million tons of human beings.

Either way, we constitute a dangerous preponderance of life on the planet.

By the way, to see if the above numbers ballpark reality, I checked the population of Washington state, currently 7.2 million, and the number of the largest wild animals, deer (about 300,000) and elk (about 60,000). This gives us some 500,000 tons of human being to about 40,000 tons of wild animal.

Washington state is also home to over a million cattle, averaging a ton each, giving us over 1.6 million tons of humans and our cattle to about 0.04 million tons of wild animal. To save time and make it look not quite so bad, let’s leave out other domesticated food animals and pets. This quickly derived ratio of 40:1 (40 pounds of humans plus cattle for each pound of wild animal) for Washington state is similar to the number given above for the world and provides some assurance that our numbers are generally representative of the facts on the ground.

For a graphic of this general concept using Vaclav Smil’s data, see

Thanks to Jody Bower and Michael Rienstra for their invaluable help editing this newsletter.



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Medicine for People! is published by Douwe Rienstra, MD at Port Townsend, Washington.