Where the tool lives and how to run it
Go to Audiences, tick the boxes next to the audiences you want to compare (up to 5), then choose Actions and Show audience overlap. The first audience you tick becomes the Selected audience and the rest are listed as Comparison audiences; you can switch the selected one from the drop-down at the top right. Meta says you only get useful results from audiences of at least 10,000 Accounts Center accounts. (Meta Business Help Center)
What the two columns mean
Overlap is the number of accounts in both audiences, and it stays the same whichever audience is selected. % overlap of selected audience divides that number by the size of the selected audience, so it changes when you swap them. Take a 1,000,000-account audience and a 100,000,000-account audience that share 800,000 accounts: that is 80% of the smaller audience but only 0.8% of the larger one. Meta's point is that the overlap is much more likely to cause delivery problems for the ad set targeting the smaller audience.
Audience overlap vs auction overlap
They are two ends of the same problem. Audience overlap is a property of your audiences, and you can check it before launch. Auction overlap is what happens in delivery when ads from your overlapping ad sets land in the same auction: Meta enters the ad with the highest total value and holds the others back, which can stop an ad set spending its budget or exiting the learning phase. Meta says overlapping audiences aren't necessarily a bad thing. They matter when they split your delivery.
What we do with the number
A high overlap between two ad sets chasing the same result is usually a reason to consolidate, and Meta's own fixes for auction overlap are combining similar ad sets or turning off the one with the fewest results and moving its budget across. We still check cost per result before switching anything off, for the reason in don't merge Meta ad sets because Meta suggests it. Overlap between a prospecting audience and a retargeting audience is a different case, where an audience exclusion is the right tool.
Common questions
Where is the audience overlap tool in Meta Ads Manager?
In Audiences. Tick the boxes next to up to 5 audiences, then choose Actions and Show audience overlap. Meta says the tool only gives useful results for audiences of at least 10,000 Accounts Center accounts.
Why does the overlap percentage change when I switch audiences?
Because the percentage is calculated against whichever audience is selected. The number of shared accounts stays the same, but 800,000 shared accounts is 80% of a 1,000,000-account audience and 0.8% of a 100,000,000-account one.
Is audience overlap the same as auction overlap?
No. Audience overlap measures how many people two audiences share. Auction overlap is what happens when your ads from overlapping ad sets enter the same auction: Meta enters only the one with the highest total value, which can leave the others underspending or stuck in learning.
Is audience overlap always bad?
No. Meta says overlapping audiences are not necessarily a bad thing, but they can lead to poor delivery. It matters most when two ad sets chase the same result, and most for the ad set targeting the smaller audience.