A conversions API is a server-side interface that lets an advertiser send conversion events, such as purchases, leads, or sign-ups, directly from its own server to an ad platform instead of relying only on a browser pixel. The best-known example is Meta's Conversions API, which connects an advertiser's server to Meta for measurement, reporting, and ad optimization.
How it works
The advertiser's backend, or a server-side tag manager, builds an event with a name, timestamp, value, and customer information, then sends it over HTTPS to the platform's endpoint. The platform matches the event to ad interactions using identifiers such as click IDs or hashed customer data. Meta states that server events are processed like events sent through its Pixel.
When a pixel and a conversions API report the same purchase, the platform has to avoid counting it twice. Meta deduplicates browser and server events by matching their event name and event ID.
Example
Example: a lead that a sales team qualifies three days after an ad click.
Day 0 User clicks a Meta ad and submits a form on brand.example.com
Day 3 CRM marks the lead "qualified"
-> server sends a "Lead" event (event ID, value, hashed email)
to the Conversions API
-> Meta can match it to the original ad interaction
A browser pixel could only report the form fill on day 0, not the qualification on day 3.
Why it matters
A conversions API is the ad-platform counterpart of an affiliate postback URL: both move conversion reporting from the browser to the server, so data survives ad blockers and cookie limits and can include events that happen later. For brands and affiliates buying paid traffic, sending qualified conversions rather than raw form fills gives the platform better optimization signals. Google Ads offers a comparable click-ID-based route through offline conversion imports; see Google Ads offline conversions for affiliates.
Related: S2S tracking, pixel tracking, click ID.