How to use the lookalike audience size calculator
One thing has to be clear first: the real size of a lookalike audience is produced by the platform. The signals it uses to judge similarity are not published, so no outside tool can compute it exactly. This calculator only converts population ร similarity percentage into an arithmetic ceiling, and the actual size must be read in your ads manager. The population figure is not invented either โ you enter it, taken from the addressable audience your ads manager reports for the country, age and language filters you have set.
The similarity percentage is the setting you choose. Meta offers a 1-10% range, where 1% is the narrowest and closest-matching slice, so it is precise and expensive. Raising the number widens reach while loosening resemblance to the seed. The multiple-of-seed row is there to gauge how far the expansion goes.
Some platforms will not build a lookalike at all when the seed list is too small. The minimum varies by platform and changes over time, so check that platform's current guidance. If you plan to exclude existing customers, enter that count to see how many genuinely new people remain.
Frequently asked questions
That is expected. This is an arithmetic ceiling from population times a percentage, while the platform narrows candidates with its own signals. Base spending decisions on the ads manager figure.
It depends on the goal rather than a right answer. Tighter percentages protect conversion efficiency; wider ones buy reach. Running both as separate ad sets and comparing results is the most reliable way to decide.