Outlook free/busy explained: what others can see, and why some calendars show as unavailable

Free/busy is the part of your Outlook calendar that says when you are busy without saying what you are doing. It is what the Scheduling Assistant shows as coloured blocks, and it is the only thing tools like OpenSlot read.

What it contains

For each interval (usually 30 minutes) a single status: free, tentative, busy, out of office, or working elsewhere. No titles, no attendees, no locations, no bodies — unless your organization has chosen to share more detail, which is separate from free/busy.

Who can see yours

By default, everyone in your Microsoft 365 organization can see your free/busy; people outside it cannot, unless your admins have set up organization relationships or you share your calendar with them individually. That is why a scheduling tool inside your company can merge colleagues' calendars instantly, but can't see a client's.

"No free/busy information could be retrieved"

The usual causes: the person is outside your organization; they have restricted free/busy sharing in Outlook (Calendar → Share → Permissions → "Can view when I'm busy" is the normal setting); their mailbox is on a different system; or the address is a distribution list, not a mailbox. Ask them to set permissions to at least "when I'm busy", or schedule by sending your own availability instead.

The string it actually arrives in

Underneath the coloured blocks, free/busy is a single string with one character per interval. Microsoft Graph calls it the availabilityView, and a request for a nine-hour day at hourly resolution comes back looking like 000220130 — one digit per hour, in order.

The interval is yours to choose. Graph documents the default as 30 minutes, with a minimum of 5 and a maximum of 1440 — a single character for a whole day. Thirty is the right default for scheduling: it matches how most people book, and it keeps a sixty-day window to a few thousand characters per person.

The digit that is documented but never appears

The mapping is 0 for free, 1 for tentative, 2 for busy and 3 for out of office. The interesting one is 4, which is reserved for working elsewhere — and which you will almost never see. Microsoft's own reference for scheduleInformation spells it out: 0 = free or working elsewhere, with the note that "working elsewhere is set to 0 instead of 4 for backward compatibility" — because Exchange 2007 and Exchange 2010 had no such status to report.

This trips people up in a specific way: someone marks themselves as working elsewhere expecting it to read as unavailable, and every scheduling tool in the organization treats them as free. That is not a bug in the tools. Working elsewhere means "working, just not here", and free/busy has always treated it as bookable. If you need the time protected, it has to be busy or out of office — a status, not a location.

There is a second quiet rule in the same area. When a tentative block and a busy block overlap the same interval, busy wins in the string. A meeting that looks provisional in the grid can be firmly taken underneath.

When the answer is too big to give

One failure has nothing to do with permissions and surprises people who hit it. When a requested window contains more than 1,000 calendar entries for a single mailbox, Microsoft returns error 5006 — "The result set contains too many calendar entries. The allowed size is 1000" — instead of a partial answer.

The mailboxes that hit this are the predictable ones: a heavily-booked meeting room, a shared support inbox, a resource calendar with a recurring booking every half hour. The fix is a shorter window rather than a different permission, which is worth knowing because the error reads like an access problem and is not one.

Rooms and lists count too

Free/busy is not only a property of people. The same lookup works for resources — meeting rooms and equipment — and for distribution lists, which is how "is the large conference room free on Thursday?" gets answered without opening the room's calendar. It is also why a failed lookup is sometimes a data-entry problem: a distribution list address that was never a mailbox has no free/busy to return, and the error looks identical to a permissions failure.

Everyone has their own working hours

Alongside the availability string, the response carries each person's working hours: which days they work, their start and end times, and the timezone those are expressed in. This is the thing most often missed when reading a grid by eye. A colleague whose row looks wide open at 8 AM may simply not start until 9:30 — their calendar is free, but the time is not really on offer, and proposing it anyway is how you get a polite decline.

Turning free/busy into a list

Reading free/busy in a grid is fine for one person; for a group across two weeks it is not. OpenSlot queries the same free/busy data for up to 40 people on Pro — two on the free plan — merges it, and hands back the gaps as text:

All times in US/Eastern
• Today 07/08
  03:30 PM - 04:00 PM
  05:00 PM - 05:30 PM
• Thursday 07/09
  09:30 AM - 10:30 AM
  12:00 PM - 01:00 PM
  02:30 PM - 03:00 PM

Common questions