Hello everyone,

Do you all have any recommended books, websites or YouTube channels on mental calculation? I would like to memorize formulas to do speed math. Thanks

I’ve been running the World Mental Calculation site for nearly 3 years, and it’s got some links to training material and advanced methods here: https://worldmentalcalculation.com/learning-training/

Mental calculation is a broad topic, with everything from speed addition with the soroban, to practical mental math like currency conversion, to advanced techniques such as calendar calculations and deep roots. Let us know if there’s anything specific you’re looking for

Thanks! I’ll check it out

I didn’t find currency conversion calculation on the site. Like calendar calculation, does currency conversion have special rules or it’s the application of the broad mental arithmetic rules? Thanks

Currency conversion is essentially just approximate division (or multiplication), and there are various methods for that, including:

- Cross-multiplication (but ignoring the least significant digits)
- Rescaling methods (adjusting both numbers by complementary factors)
- Two-step methods (e.g. divide by 5 and and add 10%)

I’ve just realized that the currency conversion pages are difficult to find on my site, so thanks for pointing that out!

Here it is: https://worldmentalcalculation.com/fast-currency-conversion-in-your-head-without-a-calculator/

For currency conversions, since one of the numbers (the rate) stays the same (more or less) my favourite approach is the two-step methods, as you can do the calculations blind without having the amount or the rate written down while you do the calculation.

The page I linked above has a generator that produces two-step rules for some currencies, so you can play with these to get an idea of how they work

You can use the same idea for unit conversions, e.g. between feet and metres.