How to Share Availability Across Multiple Calendars

The five-tab availability check

A client asks "when are you free next week?" and the ritual begins. You open the Outlook tab for client A, the Google tab for client B, your own calendar, maybe a spreadsheet, and start cross-referencing by eye. Ten minutes later you send four time slots, one of which turns out to be wrong.

The reliable way to share availability across multiple calendars is to stop doing the math yourself. Merge every calendar into one view, let software compute the actual free slots across all of them, and send a clean summary. Here is how to do that in a few minutes, without handing any tool server access to your accounts.

Why your calendars cannot answer this on their own

Free/busy sharing stops at the organization boundary. Client A's Outlook has no idea client B's Wednesday workshop exists, and neither of them knows about your dentist appointment. Each calendar is confident and each calendar is wrong.

Booking tools solve this by connecting to every account over OAuth, which works right up until a managed tenant says no. Since November 2025, Microsoft Entra's default settings block user consent to third-party apps that request calendar permissions, so connecting a client's Microsoft account usually needs an admin's approval. We wrote up what changed and why. As a contractor, asking someone else's IT department for a favor is rarely a quick errand.

manyCalendars takes a different route: everything is computed locally in your browser, so there is no account connection for anyone to approve. As far as we know it is the only calendar tool that works when IT says no.

Step by step

Step 1: Install manyCalendars and add every calendar

Install the extension from the checkout page. A 14-day full trial starts on install, no card, no account. On the Free tier you can add unlimited manual calendars as .ics files or feed URLs. For work calendars you cannot export, open the calendar in a browser tab and turn on Tab Sync, so the calendar follows that tab. Tab Sync reads what is visible in a tab you are signed into and never writes anything back to the source.

Step 2: Check the merged view is complete

Availability math is only as good as its inputs. Scan the merged week view and make sure every calendar that can block your time is present and toggled visible in the sidebar. A forgotten personal calendar is how "I'm free Thursday at 2" collides with a school pickup.

Step 3: Open Share availability

Click Share availability in the toolbar. It looks at every visible calendar, merges overlapping busy blocks, and lists the gaps. Cancelled meetings do not block time, all-day events like PTO block the whole day, and anything shorter than 15 minutes is ignored because nobody wants that meeting anyway.

Step 4: Pick the range, work hours, and time zone

Choose this week, next week, or the next two weeks. Set the work-hours window (9:00 to 5:00 by default) and pick the recipient's time zone so the slots read correctly on their end. The summary also respects your work-week days, so Saturdays never sneak in.

Step 5: Copy and paste

One click copies a plain-text summary you can paste into email, Slack, or a text message. It looks like this:

I'm available (Eastern (NYC)):
  Mon 7/6: 9am - 11:30am, 1pm - 5pm
  Tue 7/7: 9am - 10am, 2:30pm - 5pm

One detail we are proud of: it never offers the past. Slots that already started are trimmed to the next quarter hour, so you cannot send "free this morning" on a Thursday afternoon.

Honest limits, and when a booking page is better

The summary is a snapshot, not a live booking page. If something books after you hit send, the text you sent does not know. For most consultant conversations that is fine, because the reply comes back within hours and you confirm the slot anyway. If you truly need a hosted page that stays current and lets people self-book, CalendarBridge does that well (Basic $4, Premium $8, Pro $32 per user per month) with cloud-hosted two-way sync. The tradeoff is that its OAuth model connects each account server-side, and on managed Microsoft tenants that can require admin approval.

manyCalendars trades the live page for zero server access: your events never leave the browser, and you decide exactly who sees your free time and when. It is also a desktop browser extension for Chrome, Edge, and Firefox, so there is no phone app to send summaries from.

What it costs

Share availability ships in Premium, which is $29/month or $290/year ($24.17/month equivalent), alongside Privacy Mode, cross-calendar conflict detection, and auto-block busy mirroring. The 14-day trial includes all of it, so you can test the whole flow on a real client email before paying anything. Full details on the pricing page.

FAQ

Does the summary update after I send it?

No. It reflects your calendars at the moment you copy it. If the week shifts, generate a fresh one, it takes seconds.

Which calendars count toward the free slots?

Every calendar toggled visible in the sidebar. Hide a calendar and it stops blocking slots, which is useful when a feed holds reference events that should not count as busy.

How are all-day events handled?

An all-day event blocks the entire work window for that day. PTO and conference days will never leak into your offered times.