What Is event_id? (Meta Pixel + CAPI Deduplication)
event_id is a unique identifier attached to both the browser-side (Pixel) and server-side (Conversions API) copies of the same conversion event, such as a purchase.
Meta uses this shared value to recognize that the two events describe one real-world action and merges them into a single counted conversion instead of two. Without a matching event_id, Meta may count the browser and server events separately, inflating reported conversions.
- Pixel+CAPI setup
- the exact same event_id value
Daniil Maximkin Published Reviewed
- In practice
How does event_id work in practice?
When a store runs both the Meta Pixel in the browser and the Conversions API (CAPI) on the server for the same event — most commonly Purchase — both events need to carry the exact same
event_idvalue.The two events can differ in timing, payload detail, and even which parameters are present, but as long as
event_idmatches, Meta’s deduplication logic treats them as one event when deciding what to count in Ads Manager and Events Manager. This is whyevent_idis central to any Pixel+CAPI setup: it lets a merchant send richer server-side data without double-counting conversions the Pixel already reported. - Not to be confused with
Not to be confused with Is not Is something that happens automatically both the browser and server events must generate or receive the identical string event_idis not the same asevent_name(e.g., “Purchase”) or the order ID itself, though it is frequently built from one.It also isn’t something that happens automatically — both the browser and server events must generate or receive the identical string, which typically means an order ID (or a value derived from it) is pushed into the dataLayer at the moment of purchase and read by both the Pixel tag and the server-side event payload. If the two systems generate
event_idindependently — for example, the Pixel uses a random session token while CAPI uses the order ID — deduplication fails silently and conversions get overcounted. - Where to check
How to check
- Meta Events Managerpixel or datasetOverview
- Meta Events Managerpixel or datasetTest Events
- Meta Events Managerpixel or datasetDiagnostics
In Meta Events Manager, open the Overview tab for the relevant pixel or dataset and use the Test Events tool, which flags events where a browser and server source have matched on
event_id.The Diagnostics tab surfaces warnings when server and browser events for the same action arrive without a shared
event_id, which is the fastest way to catch a broken dedup setup before it skews ad reporting. - Sources
