What Is Topics API?
Topics API is a browser-based interest-targeting mechanism developed by Google as part of the Privacy Sandbox initiative. Instead of tracking individual users across websites, it assigns each browser a short list of broad interest categories derived from recent browsing history. Advertisers receive those topics during an ad auction. The user’s actual browsing data never leaves their device.
Google introduced Topics API in 2022 as a direct replacement for FLoC (Federated Learning of Cohorts), which faced regulatory and industry pushback over fingerprinting risks. Topics API is currently supported in Chrome and is the primary cohort-based targeting signal available as third-party cookie deprecation continues across the open web.
How Topics API Works
The mechanism runs entirely within the browser. Chrome observes which sites a user visits each week and maps those domains to a taxonomy of approximately 470 interest categories, such as “Travel,” “Fitness,” or “Personal Finance.” At the end of each epoch (one week), the browser selects up to five top topics that best represent the user’s browsing pattern for that period.
When a user visits a page that calls the Topics API, the browser returns up to three topics: one from each of the three most recent epochs. Each returned topic is drawn from the top five of that week, with a 5% chance of a random topic being substituted to preserve privacy. The advertiser or ad tech platform uses those topics to inform bidding decisions in a programmatic auction.
The Epoch Structure
| Epoch | Duration | Topics Stored | Topics Returned to Caller |
|---|---|---|---|
| Current week | 7 days | Top 5 | 1 (randomly selected from top 5) |
| Previous week | 7 days | Top 5 | 1 |
| Two weeks prior | 7 days | Top 5 | 1 |
Only callers that have observed a user on a page associated with a given topic receive that topic in return. This means a travel publisher is more likely to receive a “Travel” topic signal than a tech publisher with no travel-adjacent content. The API is designed so that each caller sees only what is relevant to their own inventory.
Topics API vs. Third-Party Cookies
Traditional behavioral targeting relied on third-party cookies to build detailed cross-site user profiles, often tracking hundreds of individual page visits. Topics API replaces granular URL-level data with coarse category signals for interest-based advertising. The tradeoff is significant in both directions.
- Privacy gain: No individual site visits are shared with ad tech vendors. The taxonomy is broad enough that a single topic (e.g., “Sports”) describes millions of users.
- Signal loss: Advertisers lose recency, frequency, and intent signals. Knowing a user is interested in “Autos” is far less actionable than knowing they visited three car comparison pages in the last 48 hours.
- Audience scale: Because topics are broadly defined, audience segments tend to be larger but less precise. A luxury automaker targeting “Autos” reaches car enthusiasts, mechanics, and casual readers alike.
Google’s early tests showed click-through rates for Topics-based campaigns at roughly 90% of cookie-based campaigns in some verticals, though results vary considerably by category and creative quality.
How Advertisers Access Topics API
Advertisers do not call Topics API directly. Access runs through participating demand-side platforms and ad tech intermediaries that have integrated the API into their bidding stack. When a publisher page loads, the browser checks whether the calling ad tech domain has observed the user on a relevant topic page. If yes, the API returns a topic identifier, which the bidder can use to adjust its bid price.
A simplified bid adjustment model might look like this:
Adjusted Bid = Base CPM × Topic Relevance Multiplier
For example, a financial services advertiser with a base CPM of $8.00 might apply a 1.4× multiplier when Topics API returns “Personal Finance.” That raises the effective bid to $11.20. Platforms like Google Display and Video 360 and The Trade Desk have each built Topics API support into their bidding logic, though implementation details differ by platform.
Limitations and Criticisms
Topics API has attracted criticism from multiple directions since its launch.
Granularity Concerns
The taxonomy of 470 categories is broad by design, but this creates real targeting gaps. Verticals that depend on precise audience segmentation, such as pharmaceutical advertising or B2B software, find broad categories like “Health” or “Business” insufficiently actionable. A brand running a campaign for a specific chronic condition treatment cannot use Topics API to distinguish a patient from a general health reader.
Cross-Device Gaps
Topics API is scoped to a single browser on a single device. A user who browses on both Chrome desktop and a mobile browser accumulates separate topic histories in each environment, with no mechanism to reconcile them. This limits frequency capping and sequential messaging strategies that rely on recognizing the same individual across sessions.
Publisher Dependency
Because callers only receive topics they have “observed” on relevant pages, smaller publishers in niche categories may find that their inventory returns fewer usable topic signals. A B2B cybersecurity publisher, for instance, may not appear in Chrome’s domain taxonomy mapping with sufficient frequency to consistently surface the “Cybersecurity” topic to ad callers.
Regulatory Status
The UK’s Competition and Markets Authority (CMA) reviewed Topics API as part of its broader Privacy Sandbox oversight. It required Google to commit to transparency and interoperability conditions before Chrome could proceed with full third-party cookie removal. Advertisers operating in the EU should monitor how Topics API intersects with GDPR consent requirements. Under some interpretations, even browser-side processing of browsing history may require a legal basis.
Practical Considerations for Media Buyers
Campaigns that historically performed well on audience targeting will likely experience performance variance when shifting to Topics-only inventory. A few adjustments tend to help:
- Pair Topics with contextual signals. Contextual advertising remains cookieless by nature. Combining a Topics signal with page-level contextual classification increases targeting precision without additional privacy tradeoffs.
- Expand creative variation. Broader audience segments benefit from more varied creative. A single ad unit speaking to a specific buyer persona performs poorly when the same unit reaches a much wider interest group.
- Test incrementally. Run Topics-enabled line items alongside audience-targeted line items during transition periods to measure actual performance gaps before shifting budget allocations.
- Evaluate by vertical. Topics API performs most strongly in mass-market consumer categories like Travel, Entertainment, and Retail. Niche B2B and high-intent purchase categories tend to show larger performance drops.
Frequently Asked Questions
What is the difference between Topics API and FLoC?
Topics API replaced FLoC (Federated Learning of Cohorts) in 2022. FLoC grouped users into large cohorts based on browsing behavior, which raised fingerprinting concerns because the cohort ID could be used to identify individual users. Topics API uses a fixed taxonomy of broad interest categories instead, reducing how linkable the signal is to a specific person. Google dropped FLoC after pushback from privacy researchers and regulators.
Does Topics API replace third-party cookies entirely?
No. Topics API replaces one specific use of third-party cookies: interest-based audience targeting. It does not replace frequency capping, conversion tracking, or retargeting. Those use cases require separate Privacy Sandbox APIs, including the Attribution Reporting API and the Protected Audience API.
Which browsers support Topics API?
Topics API is currently supported in Google Chrome. Safari and Firefox have not adopted it. Safari relies on Intelligent Tracking Prevention (ITP) and Firefox uses Enhanced Tracking Protection (ETP) as their respective privacy approaches. Topics API signals are only available for Chrome traffic, which limits reach in markets where Chrome is not the dominant browser.
Does Topics API require user consent under GDPR?
The consent requirements for Topics API remain unsettled and vary by jurisdiction. Under some interpretations, even browser-side processing of browsing history may require a legal basis under GDPR. Users can view and delete their topic assignments directly in Chrome settings. Advertisers operating in the EU should seek legal guidance on how Topics API fits their consent framework.
How accurate is Topics API for ad targeting?
Topics API is intentionally imprecise by design. The taxonomy of roughly 470 broad categories means audience segments are large but loosely defined. Google’s internal tests suggested Topics-based campaigns reached around 90% of the click-through rates of cookie-based campaigns in some categories, but performance varies significantly by vertical. High-intent and niche categories generally see the largest drops in targeting accuracy.