What counts as a calendar, a sync, and an account

An account is one login: one Google account, one Microsoft account, one iCloud account. A calendar is one calendar inside an account. Most people have several without realising, since Google gives you Birthdays and Holidays alongside your main one. A sync is one rule connecting calendars to each other.

Your plan limits all three separately, which is why "I'm under the calendar limit but it still won't let me" is a normal thing to run into.

The limit that actually stops people

On Free it's the sync count, not the calendars. One sync means one rule, and a rule has a single direction and a single set of settings.

So a Free plan handles "keep my work and personal calendars in step" without trouble. What it can't do is two unrelated arrangements at once — say, personal feeding into work as Busy, and a shared family calendar feeding into personal with full titles. Those are two rules, and Free has room for one.

Worth checking before you upgrade for capacity: if you're stuck at "you've reached your sync limit" with three or four calendars, sometimes one Push with several sources does what you were about to build two syncs for.

Why one sync isn't the same as one calendar pair

A sync can hold more than two calendars. Three client calendars pushing into one working calendar is one sync with four calendars in it, not three syncs.

That's the distinction the numbers above turn on: the sync count limits how many arrangements you have, the calendar counts limit how big each arrangement gets, and they run out at different times.

Calendars you didn't know you had

Connecting an account doesn't sync anything by itself. You pick calendars per sync, and only the ones you pick count against the calendar limits.

Google accounts usually carry Birthdays, Holidays, and sometimes Tasks. These show up in the picker and are easy to select by accident, at which point they eat calendar slots and fill your other calendar with birthdays. Google generates birthday entries from Contacts, so they behave oddly compared to real events. Leave them unselected unless you specifically want them.

Known limitations

• Mirror allows fewer calendars per sync than Push, on both paid plans. Four against ten on Plus; five against twenty on Pro.
• Some multi-calendar setups are refused even inside the numeric limits. A sync's total cost is weighted, not just counted, so a large Pro sync can be turned down at four calendars while a simpler one is fine at five. The error names the limit you hit.
• The same calendar can't be in two syncs that would fight over it. You'll get a conflict warning when you build the second one, which is deliberate — two syncs writing to one calendar is how you end up with duplicates.
• Downgrading doesn't delete anything immediately. You get a grace period to pick what to keep before the caps apply.