# How does the audience estimator return counts instantly while I type?

Asked by nikolai_b on 2025-12-09. Tags: performance, audiences, dx.

Purely a curiosity question! I was editing an audience today and noticed the
live estimate updates basically as fast as I can type new conditions. I do this
kind of thing for a living (data eng) and a naive scan over every profile for an
arbitrary boolean condition should take way, way longer than that.

Is it precomputed aggregates? Sampling? Some sketch structure? I tried a few
weird conditions to see if I could catch it approximating and the counts stayed
plausible every time. Genuinely curious how this is built, would love a pointer
if the team has written anything up about it

## 1 answer

### Answer from lu_moreno (2025-12-10)

You are in luck, the team wrote exactly the post you are asking for:
["Counting the uncountable: 115 days of computation to seconds"](https://blog.croct.com/post/audience-estimator)
on the engineering blog. The title is the answer to your intuition: the naive
scan really would take that long, and the write-up walks through how they got
it down to interactive speed.

Two things worth knowing as a user of the feature:

1. The live estimator is a design-time sizing aid. It exists so you can see
   whether a condition matches 40 people or 40 thousand before you commit to
   it. It is included from the Growth plan up.

2. It is not what runs in production. When the experience is live, audience
   membership is evaluated in real time per interaction, so the actual
   matching decision for each visitor is a fresh evaluation of the condition,
   not a lookup against the estimate.

So the estimate answers "how big is this audience roughly" and the runtime
answers "is this specific visitor in it right now". Different machinery for
different questions.