How to prevent double-booking across Google, Outlook and iCloud calendars

Most double-bookings come from a calendar the booking page couldn't see: the personal Gmail, the client's Outlook invite, the family iCloud calendar. This post is part of the scheduling hub and explains how the checks work and how to close the gaps.

How free/busy checks work

A booking page asks your calendar provider a narrow question: when are you busy between these two dates? Google's Calendar API answers with a freeBusy query that returns a list of busy intervals for each calendar, without titles or guests (Google). With Microsoft 365 and Outlook.com, tools read the events in a date range through Microsoft Graph's calendarView and use each event's show-as status (Microsoft Graph). The tool then removes every slot that overlaps a busy time, plus its buffers.

Whether an event blocks you depends on how it's marked, not what it says. In Google Calendar, an event is opaque (busy) by default, and "Show me as Available" makes it transparent, so it doesn't block time (Google). Outlook's statuses are free, tentative, busy, out of office, working elsewhere and unknown (Microsoft Graph). SoloWOS treats every Outlook status except free as busy, and skips canceled events.

What SoloWOS checks, and what it doesn't

Set up your calendars in SoloWOS

1. Open Scheduling > Availability & calendars and connect a Google or Microsoft account. You can add another account of either kind afterward.
2. Under "Block times when I'm busy on:", check every calendar that holds commitments: work, personal, shared family calendars you can see.
3. Under "Put new meetings on:", pick the calendar that receives new bookings, so they appear next to your other events.
4. Set your weekly hours and a buffer before and after each meeting type.
5. Book a test slot from your own booking page, then create a busy event in a connected calendar and reload: the slot should disappear.

SoloWOS only sees when you're busy, not what your events say, and it only changes the events it creates. When a booking is canceled, the event it added is removed from your calendar.

What to do about iCloud calendars

SoloWOS connects Google and Microsoft accounts only. If you keep personal commitments in iCloud, you have three options: move the calendar that matters (for example "Family") to your Google or Outlook account; add a busy placeholder event in a connected calendar whenever an iCloud event affects work hours; or set your weekly hours around fixed personal commitments like school runs, so those times are never offered.

The race-condition problem, and how it's handled

Two people can open your booking page at the same moment and pick the same slot. A booking tool has to check again when the booking is submitted, not only when the page loads. SoloWOS fetches your calendars' busy times again at that moment, re-checks the slot against current bookings inside a database transaction, and holds a unique lock per host and start time, so a second request for the same slot is refused with a message to pick another time.

One limitation to know about: if Google or Microsoft doesn't answer, SoloWOS treats that calendar as having no events rather than blocking your whole page. A provider outage is rare, but it's another reason to keep buffers on, and to reconnect quickly when the app shows an account as needing reconnection.

• Buffers absorb overruns and nearby events; see how much buffer time to leave.
• A minimum notice keeps a last-minute booking from landing on an event you added five minutes ago on your phone.
• If you also use another booking tool, connect both to the same calendars and let each write its bookings there, so each sees the other's.

Connecting a calendar is optional: without one, SoloWOS still never lets two of its own bookings overlap. If you're moving from Calendly, the Calendly comparison lists what carries over. To share a booking page clients can schedule from, Solo is $15/month with 300 bookings a month.