I got a great question from a viewer of the Intro to Architecture and Systems Design Interviews video I created (https://youtu.be/ZgdS0EUmn70). The question is: If my mental math is really weak, is it OK to whip out a calculator app?

Below is my answer, republished here so everyone can (I hope) benefit from it:

For mental math vs. busting out a calculator, I think it’s probably inconsistent from one interviewer to the next. In no cases do I think it alone could possibly cost you the interview unless you’re interviewing at some extremely mathsy place (like an algorithmic trading company, for instance).

I’ve seen a spectrum in terms of judgement towards lack of mental math. One end has people like me, who couldn’t care less if you use a calculator. I would probably award you points for self knowledge and for using tools to make you more effective.

The other end of the spectrum would be someone who is very strong in maths and who doesn’t think the problems you’re tasked with are of sufficient complexity to warrant a calculator. With an interviewer on that end of the spectrum, you might lose some love, but it wouldn’t counterbalance an overall strong performance. For instance, in hundreds of decision-making discussions, I’ve never seen lack of mental math come up as a deal breaker.

There is one caveat to all of this. If you’re using a calculator for numbers that engineers *should* know, that could hurt you*. For instance, if 2^5 comes up and you have to bang it out on your TI-83, you’re probably going to lose real points. Even as a more math-lenient interviewer, I would have some serious questions about someone if powers of two don’t seem familiar. Likewise powers of two on the big end. 2^16 and 2^32 are both important numbers that should be in your mind as a programmer.

Also sums and differences of common powers of two. If you need a calculator to sum 4096 and 4096, I would knock you down a rung in my estimation.

Hope that helps!

* Re: Numbers engineers *should* know, here are some helpful links: