Why not knowing an answer is not a disaster
Every strong candidate hits a question they cannot fully answer in a job interview. Technical roles, senior roles, and roles in specialised fields generate questions that test the boundaries of knowledge on purpose — interviewers want to see where your knowledge ends and how you handle that boundary. The candidates who perform best in these moments are not those who know everything. They are those who respond honestly and constructively rather than bluffing, freezing, or apologising excessively. How you handle not knowing something is itself important evidence about how you will perform in the role.
What to say when you do not know
There are three types of "I don't know" and each has a different best response. Type 1: You genuinely do not know and have no basis to reason toward an answer. Say so, cleanly: "I honestly do not know the answer to that. In the role, I would [research it / consult a specialist / refer to the relevant regulation]." Then briefly describe what your process would be to find the answer. This is more credible than guessing and having a wrong answer on the record. Type 2: You are not certain but can reason toward an answer. Say: "I am not certain, but based on [what you do know], I would estimate/expect that [your reasoned answer]. I would want to verify this before acting on it." Show your thinking out loud. In technical interviews, this is often what the interviewer is testing: not whether you know the fact, but whether you can reason in its direction. Type 3: You once knew and cannot recall right now. Say: "I have come across this before but I cannot recall the exact figure/specification right now. I know [related context] and I would look up the specific details as my first step."
Pivoting to what you do know
After acknowledging what you do not know, it is legitimate to pivot to what you do know: "I do not have direct experience with [specific tool], but I have used [related tool] and the principles are similar — I would expect to get productive with [specific tool] within [timeframe]." Or: "I do not know the specific regulatory requirement you are referring to, but I do know the general framework in this area [briefly describe], and I would identify the specific requirement as part of onboarding in the role." A pivot shows range and initiative rather than a dead end.
What not to do
Do not bluff. Experienced interviewers, particularly in technical or specialist roles, know when they are being given a plausible-sounding but wrong answer. Being caught bluffing is worse than not knowing. Do not over-apologise. One brief acknowledgment of the gap is fine; five sentences of apology signals lack of confidence and takes up time better spent showing what you do know. Do not say "that's a great question" and then fail to answer it. This has become a recognised stall tactic and creates a negative impression before you have even answered.