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.

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.


Before you begin

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

Send user IDs

The value you send for user_id depends on the state of the user:

  • User has never signed in: Don't send the user_id parameter.
  • User is signed-in: Send their user ID.
  • User was signed-in, then signed out: Send null.

To send a user ID to Analytics, add the user_id parameter to the config command on each page of your website:

if (/* your logic for determining if the user is signed in */) {
  gtag('config', 'TAG_ID', {
    'user_id': 'USER_ID'
  });
} else if (/* your logic for determining if the user signed out */) {
  gtag('config', 'TAG_ID', {
    'user_id': null
  });
} else {
  // Do nothing if the user never signed in.
}
  1. Replace TAG_ID with your tag ID.
  2. Replace the comments with your checks for if the user is signed-in, and if the user was signed-in but then signed out.
  3. If a user is signed-in, replace USER_ID with their user ID.
  4. When a user signs out, set user_id to null. Don't send an empty string (""), a blank string (" "), or the quoted words "null" or "NULL".
Previous
Add parameters
Next
Measure exceptions