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Microsoft 365 / Office 365 Tenant Name Check

Before you can create a new Microsoft 365 tenant, you have to pick a name, and that name becomes your permanent yourname.onmicrosoft.com address. The catch is that every tenant name has to be unique across all of Microsoft 365, so the one you want may already be taken by another organization. This tool checks that for you in a second. Type in a name and it tells you whether the .onmicrosoft.com domain is still available or already in use. It's handy when you're setting up a brand-new Microsoft 365 or Office 365 tenant, planning a tenant rename, or just confirming whether a company already has an Entra ID (Azure AD) tenant under a given name. There's no sign-in and no setup. Just enter a name and check.

.onmicrosoft.com

Letters and numbers only, no spaces or symbols. Must start with a letter, 27 characters max.

How this works

Every Microsoft 365 tenant gets a globally unique name.onmicrosoft.com domain, so a name can exist only once across all of Microsoft 365.

When you check a name, your browser asks a public Microsoft sign-in endpoint whether that tenant exists. If Microsoft returns the tenant's configuration, the name is taken; if it reports the tenant was not found, the name is available.

No sign-in, password, or private data is involved. This only reveals whether a tenant name is registered, which is public information. The check runs entirely in your browser, directly against Microsoft.

Reads only public Microsoft sign-in metadata. Not affiliated with or endorsed by Microsoft.

How to use the tenant name check

  1. Type the name you want in front of .onmicrosoft.com. Letters and numbers only, starting with a letter.
  2. Select Check.
  3. If the name is taken, try a variation. Adding a word like "corp", "group", or your industry usually gets you something usable.

What to know before you pick a tenant name

Your tenant name is more permanent than most people expect, so it's worth a minute of thought:

  • You can't rename or remove your original onmicrosoft.com domain. Microsoft keeps it tied to the tenant for good. You can add a new onmicrosoft.com domain later and make it your fallback domain, but you're limited to five of them, and the original never goes away.
  • Your SharePoint and OneDrive URLs come from it. If your tenant is contoso.onmicrosoft.com, your sites live at contoso.sharepoint.com. Changing that later is a separate PowerShell process with real limits. The standard option only works for tenants with 10,000 or fewer sites, and you can rename at most twice over the life of the tenant.
  • A custom domain doesn't replace it. You can and should add your own domain (like contoso.com) for email and sign-in, but the onmicrosoft.com name still shows up in SharePoint URLs and behind the scenes.

Microsoft's Domains FAQ and Change your SharePoint domain name have the full details.

Frequently asked questions

The name I want is taken. Can Microsoft tell me who has it?

No. Tenant names are first come, first served, and Microsoft doesn't share any information about the tenant that holds a name. If it belongs to your own organization, for example an old trial, that tenant has to be deleted before the name frees up.

Does "available" mean the name is reserved for me?

No. The check tells you the name is free right now. Someone else could still claim it before you finish signing up, so create the tenant soon after checking.

Can I use this to see if a company already has Microsoft 365?

It tells you whether a tenant exists with that exact name, which is useful when you're working with a partner or planning a migration. A company could also be using a tenant under a completely different name, so a "not found" result doesn't prove they don't have one.

Is this check private?

It only asks Microsoft's public sign-in metadata whether a tenant name exists. No sign-in, password, or private data is involved, and the request goes straight from your browser to Microsoft.

New to how SharePoint and OneDrive fit together in a fresh tenant? SharePoint vs OneDrive covers where files should live.