Practical math: Some easy conversions from metric to imperial

As a math tutor, you realize that conversions from metric to imperial are part of the grade 10 curriculum.  Let’s talk about a few that don’t need a calculator.

Even though the (Canadian) high school student grows up in a metric environment, the trades use both systems.  Moreover, the tutor likely grew up in the 70s, so still thinks as much in imperial as metric.

With a calculator, of course, you can easily convert any measurement to any other. Nowadays, you can just key a measurement into your browser and it will return the conversion.  In summer, however, such questions seem to arrive more often in everyday life – possibly when you’re not at your computer.

So, both for those in summer school, as well as those who might find these tricks useful in every day life, here are some simple conversions you can do in your head. While not exact (I think the temp conversion is), they get you within 2% of the answer.

kg to pounds:  double it, then add 10% of the answer.

example:  77 kg to pounds

step 1:  double the mass in kg:  77 times 2 = 154.

step 2:  add 10% more.  15.4 + 154= 169.4

So, 77kg is 169.4lbs.

metres to yards:  just add 10%.

55m is 55 + 5.5 or 60.5 yards.

inches to cm:  multiply by 5, then divide by 2.

4 inches = 5(4)÷2 = 10cm.

Fahrenheit to Celsius:

This conversion comes up a lot, but there is no convenient way without a calculator. You subtract 32 from the Fahrenheit, then divide by 1.8.

Example:  Convert 80F to C

step 1:  80-32=48

step 2:  48÷1.8=27 (rounded to the nearest whole degree).

So, 80F is 27C.

Here’s an irony about summer measurements:  According to Wikipedia, the Canadian football field is 110 yards, whereas the American is 100.  However, the Canadian football field is 100 m (since going from metres to yards you just add 10%).  So the American and Canadian are both 100 long in their own units.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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