3 Simple Tricks for Converting Metric Units in Your Head

Converting metric units is a snap with these tips.
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Converting units is a necessary evil. With the imperial system still in use in some parts of the world but the metric system being the go-to for science, U.S.-based scientists and students in particular need to be well-versed in how to convert between different units. Of course, you can go to the many online calculators specifically designed for the purpose (see Resources) or break out your calculator, but if you don’t need to be 100% accurate, there are some really handy tricks you can use for a rough conversion.

So the next time you need to perform a quick conversion on the fly, here are three quick and easy ways you can do for yourself.

Kilograms to Pounds

Converting a mass in kilograms (kg) to one in pounds (lb) doesn’t seem that straightforward, since 1 kg = 2.2 lbs. But there is a very simple method you can use to perform this conversion, all based on the fact that 1 kg is close to 2 pounds.

First, multiply the amount in kilograms by 2, then write down (or remember) the result and then divide your result by 10. Finally, add these two figures together to find the result.

An example calculation will make this clear: What is 40 kg in pounds? For the first stage: 40 × 2 = 80, and for the second stage: 80 ÷ 10 = 8. Finally, 80 + 8 = 88, and so 40 kg = 88 lbs. The precise result (if you use a calculator) is 88.18 lbs, but this easy answer is close enough for most purposes.

Inches to Centimeters

One of the most common conversions you might need to do is between inches and centimeters (cm), and this can be made pretty simple if you simplify the process a little and work with whole inches. The precise conversion is 1 inch = 2.54 cm, but you can make the conversion manageable if you calculate based on 1 inch = 2.5 cm. There are also two simple approaches you can use, one similar to the previous method and one that’s easier in some cases.

The first approach is to take your value in inches and multiply it by 2. Then divide the original number by 2, and add the two results together. Alternatively, you can multiply the number in inches by 10, and then divide the result by 4.

Imagine you have to convert 20 inches to cm. Both methods are quite straightforward in this case. For example, 20 × 2 = 40, and 20 ÷ 2 = 10, then 40 + 10 = 50, so 20 inches = 50 cm. The alternative approach gives: 20 × 10 = 200, and 200 ÷ 4 = 50 cm. The actual result is closer to 51 cm, but for a rough calculation, this is an easy approach.

Miles to Kilometers with Fibonacci

If you’re driving in another country, you’ll probably encounter many road signs and distances written in kilometers (km) instead of miles, but luckily there’s a very easy way to convert between miles and km. This is based around the Fibonacci sequence, which is a list of numbers you generate by starting at 1 and adding the current number to the previous one to get the next one. So the sequence goes 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 and so on, with the repeated 1 coming from adding 0 to 1. You can just work this out rather than remembering it:


0 + 1 = 1

1 + 1 = 2

1 + 2 = 3

2 + 3 = 5

3 + 5 = 8

5 + 8 = 13 and so on.

Because 1 mile is about 1.6 km, and each Fibonacci number is around 1.6 times the previous one, you can get an impressively accurate result for miles to km using the sequence. For example, 5 miles is about 8 km (8.05 exactly), and 8 miles is about 13 km (12.87 exactly). And you can work backwards in the sequence if you want to convert from km to miles.

Finally, if the number you need to convert isn’t in the sequence, you can do it in pieces. For example, if you want to convert 29 miles to km, note that 29 = 21 + 8, and both of these are Fibonacci numbers. So convert each (21 miles = 34 km and 8 miles = 13 km), then add the results together to find your answer (34 + 13 = 47 km).

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