How the estimate works
Visitors in a window of 30 days or less ≈ monthly visitors × (window ÷ 30)
Longer windows ≈ monthly visitors + monthly visitors × (1 − repeat share) × (extra months)
Matched audience ≈ visitors in window × match rate
Weekly budget ≈ matched audience × impressions per person ÷ 1,000 × CPM
The repeat-visitor share stops people who come back in later months being counted twice. The result is an order-of-magnitude estimate. Meta’s own audience size in Ads Manager is the figure to trust once your audience is built.
Why match rates are lower than you expect
- Visitors who decline cookies are not added if your consent setup blocks the Meta pixel until consent (as it should, where consent is required).
- Ad blockers and browser tracking protection stop the pixel firing.
- Some visitors do not use Facebook or Instagram, or are logged out.
Server-side events through the Conversions API can recover some lost signal for consented users, but not all.
Practical guidance
- Under about 1,000 matched people: retargeting will have little delivery and tires quickly. Consider a longer window, or include engaged Facebook and Instagram users as well as website visitors.
- Service businesses: use 30–90 day windows; buying decisions take longer. Exclude people who already enquired.
- Rotate creative: warm audiences see your ads often. Mix proof (reviews, results), objection-handling and offer reminders. Track it with the Frequency & Fatigue Checker.
More detail in Meta retargeting for service businesses.