What's the Minimum Source Audience Size for a Meta Lookalike Audience?

Meta's Marketing API documentation requires a source Custom Audience to contain at least 100 people from a single country before it can be used to build a Lookalike Audience. This applies to both website Custom Audiences and mobile app Custom Audiences — the 100-person floor is the documented requirement, even though the practical recommendation for audience quality runs far higher.

The Official Requirement vs. Real-World Enforcement

Meta's own developer documentation states the 100-person-per-country minimum directly. In practice, though, that floor isn't always strictly enforced — independent testing by Facebook ads practitioner Jon Loomer found a Lookalike built from a source audience of fewer than 10 people was created without the platform throwing an error, suggesting the stated minimum functions more as official guidance than a hard technical gate in every case. Treating 100 as the real floor to plan around is still the safer approach, since Meta's documentation is the authoritative source and enforcement behavior isn't guaranteed to stay permissive.

The "Single Country" Condition

The 100-person threshold specifically requires those people to come from one country — a source audience spread thinly across 15 countries with only a handful of people in each wouldn't satisfy the requirement even if the total list exceeds 100. Meta's API does offer a workaround: setting the "allow international seeds" option lets the platform pull additional similar profiles from other countries if the target country alone doesn't have 100 matched people, rather than blocking Lookalike creation outright.

Why 100 Is a Floor, Not a Target

Reaching the minimum doesn't mean you'll get a useful Lookalike. Third-party agency guidance commonly recommends source audiences in the 1,000-to-50,000-person range, since a Lookalike's quality depends on the algorithm having enough signal to find meaningful shared traits — a 100-person source audience gives Meta very little to generalize from, even if it technically clears the bar to generate output. A Custom Audience built from a single week of a low-traffic website, for example, might technically hit 100 people but still produce a Lookalike that performs poorly because the source itself was too thin to represent a real customer pattern.

Lookalikes Take Time to Populate

Meta's developer documentation also notes that newly created Lookalike Audiences take roughly 1 to 6 hours to fully populate after creation, rather than being immediately ready to serve ads at full size. Launching a campaign against a brand-new Lookalike without accounting for that population window can produce artificially low initial reach that has nothing to do with targeting quality.

Bottom Line

Build Lookalikes from source Custom Audiences well above the 100-person technical minimum — aim for several hundred to a few thousand people from one country where possible — and give a newly created Lookalike a few hours to fully populate before judging its reach or performance.

Sources: Meta for Developers, Marketing API documentation, "Lookalike Audiences" guide (developers.facebook.com); Jon Loomer, independent testing on Lookalike Audience minimum enforcement (jonloomer.com).