Unlike the quiet version of "I don't know," this one happens live, in front of a room, with no time to prepare a graceful sentence. That's exactly why it's worth having a default response ready before it happens, so you're not improvising one under pressure.
The pattern that works has three parts, said calmly and without rushing: acknowledge the question is a good one if it genuinely is, state plainly that you don't have the answer right now, and commit to a specific way and time you'll follow up. Skipping any of the three, especially the last one, is what makes this moment feel unresolved instead of handled.
"That's a fair question, and I don't want to guess at the answer, let me pull the actual number and get back to you by end of day rather than give you something that might be wrong."
That response does real work: it validates the question without over-praising it, it's honest about the gap without dwelling on it, and it gives the room a concrete commitment instead of a vague "I'll look into it." A room almost never holds a confident "I don't know, here's when you'll have it" against the presenter, what erodes trust is a fumbled guess that turns out to be wrong.
The room doesn't expect you to know everything. It expects you to handle not knowing something without losing your footing.
Resist the specific temptation to guess out loud while thinking, hoping to land on something close enough, "well, I think it's probably around... maybe 15%, or it could be higher..." This kind of live improvisation is exactly what gets remembered as the wrong number, quoted back to you later, even though you never actually claimed to know it for certain.
If the question is genuinely someone else's area, redirecting cleanly is better than pretending it's yours to answer: "that's really [name]'s area, [name], do you have a quick take, or should we follow up after?" This isn't passing the buck; it's routing the question to whoever can actually answer it well, which serves the room better than a shaky answer from the wrong person.
Whatever you commit to following up on, actually follow up, ideally sooner than promised. A presenter who says "I'll get back to you" and then does, reliably and quickly, builds more credibility from that one exchange than they would have if they'd happened to know the answer in the moment.