How important is it to be able to do mental calculations? That’s in your head. I know that there are people that can work out crazy things like 123⁷³ in their heads, but I am not one of those people. Why do companies still ask for candidates to perform mental calculations in job interviews or assessments?

I am a mathematician (that is, I have a masters degree in mathematics) and non-mathematicians often expect that I should be able to add up or divide the bill when we are in a restaurant. Then I need to explain that, actually, mathematics is not about numbers (I hardly ever see actual concrete numbers) and that I was probably going to make a mistake.

I get fluttered and nervous when I have to do mental arithmetic, particularly if other people are waiting for the answer. (I find dividing be three, or any other odd number, especially hard). The more real maths I have done, the more I have lost my ability to do calculations in my head. I consider concepts far more important. We have calculators and computers for a reason. I like to leave the calculations to those devices.

I have a distinct bias in expressing the above opinion, exactly because I am not good at mental calculations. I want to justify my inability to do such seemingly simple things. I want to prove that it is not important, that my other skills are far more valuable, that spending my time improving my mental calculation skills would be wasteful. Does it matter that my mental arithmetic is poor? Will I ever find myself in a (non-contrived) situation where it is actually important?

I was recently asked to do a simple calculation at a job interview and I could not get the right answer at first (in fact, I was so obviously wrong, that my interviewers may have doubted my credentials). Embarrassed, I got the correct answer later, when my mind had calmed down.

I failed a McKinsey entrance test which relied on being able to make mental approximations to a number of arithmetic problems in a very limited amount of time. Even though I understood most of the concepts involved, I could not perform these mental calculations at the required speed. I could have practiced such calculations for a month or two (or more) beforehand and (probably) have passed, but why? Hypothetically, some client might ask what the profit would be if x and y had certain values, but if the answer were important it would need to be arrived it far more elaborately and placed in some important document. In any case, I don’t particularly mind not having quick answers to such questions. I’d rather give you a thoroughly considered answer.

To me, mental calculations are a party-trick. It’s worthless unless you understand why you’re making the calculations and for any “important” work you’re going to be dealing with excel or computer models or in any case. I would rather spend my time improving my understanding of the concepts and how to use the computers that will do the number-crunching for me. In an age where computing power is cheap, but creativity paramount, this seems the right strategy.