Moving to the US? It's high time you awakened the mental math wizard in you

Contributed by: Swagata Bhattacharyya

When we moved to the US from India a few years back, little did I imagine that there would be so much of mathematics involved in my daily existence. Converting not just Dollars to Rupees but Kilograms to Pounds, Celsius to Fahrenheit, Kilometres to Miles, the list is endless...all the units of measurements in the US are different from the ones I grew up with. That sounds like a lot of conversions...isn't it? Till the time you get used to living with the new units of measurement and are able to work everything out lightning-fast, you would notice that conversions are required everywhere – be it talking to people or at the grocery store or travelling.

While talking to new people who you don’t know well, talking about the weather is generally considered the safest bet but for me even that became a challenge. E.g. if someone said “I went to Phoenix, the temperature there was 100 degrees”, I did not know whether to respond by saying “Ooh! That’s terrible” or “Ah! That’s pleasant”. You cannot snap out a calculator during conversations, convert and then respond, so I had to manage with a vague “Hmm” which actually meant “I don’t have any idea what you are talking about” :)

Later I gathered some quick ways to convert the most commonly used units in my head which helped me a lot to sail through the initial days. Wondering how you might work out those multiplications and conversions in a jiffy? Great news - you can do it very easily! You might read on to find out how. Here is my collection:

Pound to KG
1 pound = 0.453592 KG

Quick conversion: Divide pounds by 2 and then subtract 10%
e.g. 20 pounds: 20/2 = 10, 10 – 1 (10% of 10) = 9 KG
The actual conversion is 9.07185 KG which is close to 9 KG

For KG to Pound, double the KG value and add 10%
e.g. 10 KG = 10*2 + 2 = 22 (~22.0462 which is the actual conversion)

Fahrenheit to Celsius
The formula is
C = (F-32) * 5/9

Quick conversion: Subtract 30 from the temperature and then divide by 2
e.g. 74 F: 74 – 30 = 44. 44/2 = 22
The actual conversion is 23.33, so you are close.

For Celsius to Fahrenheit, calculate it backwards. i.e. multiply Celsius by 2 and add 30.
e.g. 22 C = 22 * 2 + 30 = 74 F (actual conversion is 71.6 F)

Miles to Kilometres
1 Mile = 1.60934 KM

i.e. you will have to add 60% to the mile value to get its KM equivalent. It is easier to do it without a calculator if you break it down to 50% + 10%
e.g. 60 Miles = 60 + 30 (50% of 60) + 6 (10% of 60) = 96 KM
The actual conversion is 96.5606 KM

Kilometres to Miles, it’s just the opposite, i.e. you are subtracting 40%. Again it is easier to break it down by subtracting 50% and then adding 10% back.
e.g. 60 KM = 60 – 30 + 6 = 36 miles(~37.2823 miles which is the actual value)

Alternate method: Another quick way to do it is to use the Fibonacci number series i.e. 0,1,1,2,3,5,8.....
This also gives a very rough estimate. So 3 miles would be ~5 km and 5 miles would be ~8km and so on.
The logic behind this is that the ratio of successive Fibonacci numbers is roughly 1.618 (which is close to our conversion ratio 1.60934)

Gallons to Litres
1 US Gallon = 3.78541 Litres

Quick conversion: Calculate by assuming 1 US Gallon = 4 litres. For a more accurate value, subtract 1 gallon for every four litres.
e.g. 12 gallon = 12 * 4 – 3 = 45 litres
The actual converted amount is 45.4249 litres.

For Litres to Gallons, divide by 4 to get the approximate value.
1 Litre = 0.264172 US Gallon
e.g. 40 Litres = 40/4 = 10 US Gallons. The actual value is 10.5669 US Gallons.


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