# Set custom User IDs

To enable seamless, personalized communication across all platforms and devices, it is essential to assign a custom User ID to each user. 

## What is a User ID?

User ID is a unique identifier assigned to each user. A single User ID can be linked to multiple devices, where each contact method (mobile app, email address, phone number) is [treated as a separate device](/product/pushwoosh-knowledge-hub/omnichannel-campaigns/#understanding-how-pushwoosh-handles-devices). 

Linking all devices (contact methods) under one User ID allows you to reach users across channels and coordinate messaging in [omnichannel campaigns](/product/pushwoosh-knowledge-hub/omnichannel-campaigns/).

<Aside type="note">
Each user can have up to **20 devices** linked to their User ID. If a new device is registered beyond this limit, the oldest inactive device is automatically removed. [Learn more](/product/pushwoosh-knowledge-hub/users-userids/#device-limit-per-user)
</Aside>

> For a detailed explanation of how User IDs work and how they relate to devices, see [this guide](/product/pushwoosh-knowledge-hub/users-userids/).

## Why you need to set a custom User ID

By default, when your app is first launched, Pushwoosh creates a User ID equal to the device’s Hardware ID (HWID). This means each device is treated as a separate user. So if one person uses more than one device, like a phone and a tablet, Pushwoosh sees them as two separate users, and their data won’t be connected.

Setting a custom User ID lets you link all devices and contact methods under one profile. This provides several important benefits:

### Accurate user counts for billing

Setting a custom User ID helps avoid duplicate user records. This keeps your account data cleaner for billing and usage review.

> [Learn more about Pushwoosh pricing](https://pushwoosh.com/pricing)

If you don’t set a custom User ID, each device can be treated as a separate user, even if they belong to the same person. For example, if one user uses both a mobile phone and a tablet, they can be counted as two users.

### Consistent messaging across channels

For a unified experience across push notifications, email, SMS, in-app, WhatsApp, and other channels, all user data must be linked under a single User ID.

By setting a custom User ID, you ensure that Pushwoosh recognizes all contact methods (devices, emails, phone numbers) as belonging to the same user, which is essential for consistent and personalized omnichannel campaigns.

> Learn more about [Omnichannel campaigns](/product/pushwoosh-knowledge-hub/omnichannel-campaigns/)

### Persistent user profiles

A custom User ID allows Pushwoosh to maintain a complete, unified profile for each user. This profile includes:

* All devices and contact info linked to the same user   
* Tags   
* Event history and campaign participation  
* Messages sent and opened

You can view and manage this data in [User Explorer](/product/audience-data-and-segmentation/user-explorer/), helping you deliver more accurate targeting and better personalization.

## How to set a custom User ID

<Aside type="caution" icon="setting" title="Developer assistance needed">
Setting a custom User ID requires assistance from your development team. Please share the following technical guide with them for implementation steps: [Collecting User IDs](/developer/first-steps/collecting-user-ids/)

</Aside>