Whether you’re travelling abroad or shopping online, it can be helpful to quickly convert between currencies to understand the prices. However, the exchange rates are often awkward numbers that are difficult for mental math!
For example, today the exchange rate from Danish Krone to Euro is 7.4571. If something costs DKK 60 then to convert that to EUR you need to divide 60 ÷ 7.4571, which would be very difficult to calculate with mental math, even for experienced and competitive mental calculators.
However there are many neat rules that you can use to make it easy to convert between currencies in your head, whether or not you are well-trained at mental arithmetic!
Let’s take our Danish example again. For this exchange rate, we can quickly use the following easy 2-step rule without a calculator:
Add on 1/3 of the original amount, and divide by 10
- 1/3 of 60 is 20 – add that on to get 80
- 80 ÷ 10 = €8
This is super close to the exact answer of €8.05, and probably more than good enough to decide whether the taxi fare is reasonable! The difficulty now is how to create a suitable rule to convert currencies in your head, so to help you I have two resources:
#1: The tool below is the Currency Math Rules Generator, which takes today’s exchange rate for over 30 currencies and suggests simple ways that you can convert from one to the other without a calculator. Try it out!
Accuracy of each rule is in brackets so e.g. (1.2%) means the rule is inaccurate by only 1.2%.
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#2: To learn about how to create rules like this, and other neat skills for practical mental math such as:
- unit conversion
- currency conversion
- re-scaling methods for approximation
- mathematical facts for instant calculation
Preview the Udemy video series Top Mental Math Shortcuts for Practical Everyday Use written by me (Daniel Timms) and available discounted to under $15 through this link. (If the discount doesn’t appear, contact me for a fresh coupon).
