Three things people mean by "merge"
Someone asking how to merge multiple calendars is usually choosing between three different mechanisms without knowing it. One is overlaying calendars inside a single app, which changes only what you see. Another is subscribing to a link, which is read-only and refreshes on the recipient's schedule rather than yours. The third is copying events between accounts, where real events are written into the destination calendar. Only the third makes you appear busy to someone looking at your other calendar, so pick it when the goal is fewer double-bookings rather than a tidier view.
What does it actually mean to merge multiple calendars?
A combined view stacks separate calendars inside one app and changes nothing about the underlying accounts — the events still belong where they always did. A subscription is a one-way, read-only feed at a URL: the destination app fetches that file, and nothing you do in it writes back. A copy writes a real event object into the destination calendar, owned by that calendar the same way an event you typed there is.
When you merge multiple calendars, this is the choice that decides everything else: only a copy exists outside the app you are looking at. A combined view and a subscription leave no trace in the underlying account, so nothing querying that account can see them. Copied events exist for anyone else reading that calendar — a colleague checking your free/busy, or a booking tool that would otherwise offer the slot.
A one-time import, meaning an uploaded .ics file, is a copy that never updates again. Google states it directly: "Imported events don't stay in sync between your 2 accounts. If you want your calendars to sync, share your calendar with the other account."
How do you merge Google calendars into one view?
Google Calendar has four separate screens for this: sharing with a person, subscribing to a URL, publishing your own iCal address, and a one-time import. To share a calendar with another account, go to My calendars, hover the calendar, then More → Settings and sharing → Shared with → Add people and groups, enter the address, and click Send. The permission levels are "See only free/busy (hide details)", "See all event details", "Make changes (see private events as free/busy)", "Make changes and see all event details", and "Make changes and manage sharing".
To subscribe to a link, use the left panel: next to "Other calendars" click Add other calendars → From URL, enter the URL, then Add calendar.
For a link to your own calendar, open Settings, click the calendar under "Settings for my calendars", open Integrate calendar, and copy the "Secret address in iCal format". Google documents a repair if it escapes: "If you accidentally shared your calendar's Secret Address, click Reset to create a new Secret Address."
For a one-time import, it is Settings → Import & Export → Select file from your computer.
Sharing overlays your calendar in their app — read-only at the free/busy and details levels, writable at the three Make changes levels. A subscribed URL is a read-only feed that their app re-reads on its own schedule. An import is a copy — real events in the destination calendar that never update again.
How do you merge calendars in Outlook?
Three screens in Outlook do the combining: Share calendar for handing access to a person, Subscribe from web for a link, and Upload from file for a one-time .ics. Share calendar is the only one that keeps showing your calendar as it changes; the other two are a subscription and a file.
To share, go to the navigation pane → Calendar → Home → Share calendar, pick the calendar, enter the person, choose the level from the dropdown, then Share. The levels are "Can view when I'm busy", "Can view titles and locations", and "Can view all details". Edit and delegate permissions live in Microsoft's separate delegation article and are out of scope here.
To subscribe to a link: Calendar → Add calendar → Subscribe from web, paste the address, then Import → Save. Outlook re-reads that address on its own schedule, not yours.
To bring in a file: Calendar → Add calendar → Upload from file → Browse, choose the .ics, Open, pick the target under Select a calendar, then Import → Save.
How fast does a subscribed calendar update?
Roughly every six hours on Outlook, and sometimes much longer. Microsoft's current support page for importing or subscribing to a calendar in Outlook.com and Outlook on the web says updates "should happen approximately every 6 hours," and adds that "this update can take more than 24 hours" (Microsoft Support). Google's help pages for calendars added from a URL do not state a refresh interval, so treat that one as undocumented rather than assuming a number.
The mechanism explains why: the subscribing app decides when to re-read the link, so whoever publishes it has no way to make the app ask sooner. That holds for Google's own secret iCal address, for any ICS URL you paste in, and for a Keeper.sh iCal feed alike — Keeper.sh rebuilds the feed on every read and serves it with an ETag, returning a 304 when nothing has changed, so an app that polls often is not penalised for it. Rebuilding only helps when something actually reads it.
When do you need to copy events between calendars instead?
Copy events when someone other than you has to see the time as taken — a colleague scanning your work calendar for a free slot, a booking page offering hours on your behalf, or a Free/Busy lookup run by your employer's system. That is the deciding question. A combined view lives inside the app on your screen and leaves the underlying account untouched, and a subscription lands in the subscribing app on its own schedule rather than in the calendar those other systems query. Only a written event answers the question with a yes.
A copy is a real event object in the destination calendar, indistinguishable from one you typed there. That helps with speed as well as visibility: once the event exists, Google or Outlook distributes it through their own native sync, at the speed they move any other event in that account, with no subscription poll anywhere in the path.
The trade-off is plain. The event now exists twice, once in each calendar, and each copy is a real event in a real calendar.
Which method should you use?
Pick by who needs to see the time — that question separates the three ways to merge multiple calendars covered above.
| Method | What it changes | Who else can see it | How fast it updates |
|---|---|---|---|
| Combined view | Only your display | Nobody | Immediately |
| Subscribed link | Adds a read-only feed in one app | Whoever holds the link | Subscriber's schedule; Outlook says roughly every 6 hours, sometimes more than 24 |
| Copied events | Writes real events into the destination calendar | Anyone reading that calendar, including booking tools | Native provider sync; Keeper.sh Pro lands Google and Outlook changes within seconds, Free within 30 minutes |
Same account, only you look at it → combined view. Someone outside needs occasional read access → subscribe. Anything that books time against you → copy.
How do you sync events between two calendar accounts?
Connect the account holding the events first, then open that calendar in the Keeper.sh dashboard, find the "Send Events to Calendars" section, and tick the calendars that should receive copies. Keeper.sh is open source and self-hostable, and self-hosted deployments get Pro behaviour for everyone.
The timing belongs to the sync itself, not to any feed. Source calendars are read every minute on both plans. Free writes into destination calendars every 30 minutes; Pro writes every minute, and changes on Google and Outlook sources land within seconds because those providers push them to Keeper.sh. CalDAV and iCloud sources have no push channel, so they stay on the one-minute timer on both plans.
Copies are private by default: event name, description and location are all excluded, and the copy shows in the destination under a placeholder title. Each is a per-calendar Sync Settings toggle, and turning any of them on is a Pro feature.
Free covers 2 accounts and 3 connections. Pro is $5 per month, or $42 per year if you pay annually.
Undo is one tick box: untick the calendar under Send Events to Calendars and new events stop being copied. Events already copied stay where they are, so clear those out yourself if you do not want them.