How to sync two Outlook calendars, or several Microsoft 365 tenants

If you work for more than one organization, you probably have more than one Outlook calendar: your own company's Microsoft 365, an account in each client's tenant, maybe a personal Outlook.com. Each client books you in the calendar they can see, and none of them knows about the others.

Outlook has three free ways to bring calendars together. They all help you see your week. None of them makes one account show you as busy when you are booked in another.

Method 1: View the calendars side by side (free)

If all your accounts are added to Outlook, you can look at their calendars together. Microsoft's steps:

• New Outlook for Windows. In the calendar list, select the other calendars you want next to your default one. In the Arrange group, select Split view to see them side by side, or clear Split view to see them in one grid.
• Outlook on the web. Open each calendar, then use the view choice in the upper-right corner of the calendar. Merge shows them together, Split shows them side by side.

This is fine for planning your own day. It changes nothing in the calendars themselves, so a colleague at client A still sees you as free during your meeting at client B.

Method 2: Share your calendar with your other account (free)

In Outlook on the web:

1. Open Calendar and select Share on the toolbar, then pick the calendar.
2. Enter the address of your other account.
3. Pick a permission. "Can view when I'm busy" shows the time only. "Can view titles and locations" and "Can view all details" show more.
4. Select Share. The other account gets an invitation and can add your calendar next to its own.

Two limits from Microsoft's help pages matter for consultants. Sharing "may be limited to people in your organization, depending on your organization settings", and classic Outlook shows "This calendar can't be shared" when the address belongs to someone outside your organization. A client's IT team often turns external sharing off. And the person you share with needs a Microsoft 365 or Outlook.com account to accept.

Even when it works, sharing gives the other account a view. Nothing is added to its calendar, so the Scheduling Assistant in that tenant still shows you as free.

Method 3: Publish an ICS link and subscribe to it (free, read-only)

Publishing works across tenants, because the other account just reads a link.

1. In Outlook on the web, open Calendar, then Settings, then Shared calendars.
2. Under "Publish a calendar", choose the calendar and how much detail to include, for example "Can view when I'm busy".
3. Select Publish and copy the ICS link.
4. In the other account's Outlook calendar, select "Add calendar", then "Subscribe from web", and paste the link.

The subscribed calendar is read-only. Microsoft says "how often your ICS calendar syncs depends on the email provider" of the account that subscribes, so changes can take a while. It also shows up as a separate calendar in the other account, so it still does not make you busy there. Anyone who gets hold of the link can read the calendar, and some organizations turn publishing off.

Method 4: Two-way sync with Caltsu

Caltsu connects each Microsoft account with its own sign-in and writes Busy blocks between them. It works across any number of tenants, because it never relies on one organization sharing with another.

1. Start a trial. It runs 14 days and does not ask for a card.
2. Choose Connect account, pick Microsoft and sign in with your first account on Microsoft's own page.
3. Repeat for each client tenant and for Outlook.com. Google accounts can join the same group.
4. Put the calendars in one sync group.

Every calendar in the group now gets a Busy block for each event in the others. The block has the start and end time, the title Busy, which you can change, and a note naming the source calendar. Client A never sees what your meeting at client B is about. Changes arrive through Microsoft Graph change notifications.

A few things to know before you connect a client account:

• Caltsu syncs the default calendar of each Microsoft account.
• Each block names its source calendar. If a calendar's name would tell one client about another, rename it first.
• Some tenants only let admins approve new apps. Send the client's IT team how to approve Caltsu in Microsoft Entra. Caltsu asks for delegated calendar access only, no mail, files or directory. The Outlook integration page lists each permission.

Caltsu is €5 a month or €48 a year, with no limit on connected accounts.

Common questions

Can I merge two Outlook calendars into one?

Not as a live merge. Outlook can show several calendars in one grid, as in method 1, and you can import a calendar file into another calendar, but an import is a one-time copy. To keep two calendars in step, something has to write each change into the other calendar. That is what a sync tool does.

Why doesn't a shared calendar block my time?

A shared or subscribed calendar is a view of another calendar. When a colleague checks your availability in the Scheduling Assistant, Outlook reads only your own calendar in that tenant. The time has to be blocked in that calendar.

Does this work with a client account I don't control?

Methods 1 and 4 need only your own sign-in to the account, though a client tenant may require admin approval before any outside app connects. Methods 2 and 3 depend on the client allowing external sharing or publishing.

Where does Caltsu keep my data?

Caltsu's database is in Frankfurt, Germany. It does not store event descriptions, attendees, locations or meeting links. Titles are kept only inside a change fingerprint that Caltsu uses to notice edits, and are never written to another calendar. See security for the full list.

I also use Google Calendar. Does the same setup work?

Yes. Connect the Google account to the same sync group. See how to sync Google Calendar with Outlook for the free and paid options, or how to sync two Google calendars if you have more than one Google account.