Send user IDs

User IDs are your own unique identifiers that you assign to individual users. This guide explains how to send user IDs to Google Analytics, so you can connect user behavior across different sessions, devices, and platforms.

To learn more about the User-ID feature, see Measure activity across platforms. To learn how to set a user ID for an app, see Set a user ID.

To avoid issues with your reporting and data accuracy, make sure you follow these best practices:

Don't set custom dimensions based on user IDs. Setting custom dimensions based on user IDs leads to dimensions with too many unique values. Having too many unique values causes issues with Google Analytics data and reporting accuracy. Learn more about best practices for setting custom dimensions.
Don't send a user ID when a user isn't signed in. If a user isn't signed in, don't pass the user ID parameter, or leave the value as undefined. Don't set a custom value, like "empty" or "NA". Setting the same value for all sessions without a user signed in causes issues with the accuracy of your reporting.

Before you begin

Before you can send user IDs, make sure you've completed the following:

Create a Google Analytics account and property.
Create a web data stream for your website.
Have the Editor role (or above) to the Google Analytics account.