The key is to remember that assumptions are not judged on exact precision. They are judged on whether they are reasonable, transparent, and internally consistent.

A strong candidate states the assumption, gives a quick rationale, and moves on. For example, you might say, ‘I will assume X because of Y, and I can test the sensitivity later if needed.’ That sounds much more credible than presenting the number as magic.

You also improve when you build a habit of anchoring assumptions to everyday reference points. That makes them easier to generate and defend under pressure. You may also find it helpful to read market sizing if you want a more structured way to practice this.

I think confidence matters a lot here. Interviewers know you are estimating, they just do not want random unexplained numbers.

Were your assumptions off by a lot, or were you mainly worried that you could not justify them quickly enough?

I usually struggle more with knowing where to start than with the math itself.

Sensitivity checks help too if you are unsure which assumption matters most.