How to Sync Multiple Google Calendars for Free Using Google Apps Script

Why Calendar Syncing is a Challenge

If you manage multiple calendars—whether for different clients, projects, or businesses—you’ve likely faced the frustration of keeping them in sync.

Many professionals use multiple Google Calendars, but when a client provides you with a company email, their team often needs visibility into your availability. The challenge? Google Calendar doesn’t offer a built-in way to sync events across accounts—and manually updating them is time-consuming.

Existing Solutions (And Their Downsides)

Several paid tools help sync calendars, including:

• OneCal
• CalendarBridge
• DDBM (Don'tDoubleBookMe)

While these tools work well, they all charge a fee for what seems like a basic function: copying events between calendars.

For those who prefer a free and customizable solution, Google Apps Script provides an alternative—allowing you to automatically sync events both ways between multiple calendars.

How to Sync Google Calendars for Free Using Google Apps Script

With Google Apps Script, you can:

✔️ Sync events between multiple Google Calendars
✔️ Reflect changes automatically (so updates in one calendar appear in another)
✔️ Remove outdated events if they’ve changed after syncing

This method requires no third-party software—just a simple script running within your Google account.

Prerequisites - Sharing permissions

In your primary calendar, you must first request permissions to access / view the secondary calendar.

You will need to: 

• Go to your Google Calendar Settings
• Add Calendar > Subscribe to Calendar
• Add the email of the calendar you want to get access to
• Your secondary calendar will receive an email asking for the permissions

Once you grant the permissions you can run the script -- see below for the code

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The Code: Google Script for Two-Way Calendar Sync

Here’s a basic Google Apps Script you can use to sync events between two Google Calendars:

function sync() {
  // Array of secondary calendar IDs – these are the calendars you’re pulling free/busy info from
  var secondaryCalendarIds = ["secondary_cal1@domain.com", "secondary_cal2@domain.com"]; // these are the calendars you pull events from

  var today = new Date();
  var enddate = new Date();
  enddate.setDate(today.getDate() + 1); // one day outlook for debugging

  Logger.log("=== Sync Started ===");
  Logger.log("Time range: " + today + " to " + enddate);

  // Use PropertiesService to track which events (by their unique time key) have been processed.
  var props = PropertiesService.getScriptProperties();
  var processedJSON = props.getProperty('processedEvents');
  var processed = processedJSON ? JSON.parse(processedJSON) : {};

  // Combine free/busy (secondary) events from all secondary calendars into one array
  var secondaryEvents = [];
  for (var i = 0; i < secondaryCalendarIds.length; i++) {
    var calId = secondaryCalendarIds[i];
    Logger.log("Fetching events from secondary calendar: " + calId);
    var cal = CalendarApp.getCalendarById(calId);
    if (cal) {
      try {
        var events = cal.getEvents(today, enddate);
        Logger.log("Fetched " + events.length + " events from " + calId);
        secondaryEvents = secondaryEvents.concat(events);
      } catch (e) {
        Logger.log("Error fetching events from calendar " + calId + ": " + e);
      }
    } else {
      Logger.log("Calendar with ID " + calId + " not found.");
    }
  }

  // Get events from primary calendar (the calendar on the account where the script runs)
  var primaryCal = CalendarApp.getDefaultCalendar();
  var primaryEvents = [];
  try {
    primaryEvents = primaryCal.getEvents(today, enddate);
    Logger.log("Fetched " + primaryEvents.length + " events from primary calendar");
  } catch (e) {
    Logger.log("Error fetching events from primary calendar: " + e);
  }

  // Process each event from secondary calendars
  for (var i = 0; i < secondaryEvents.length; i++) {
    var secEvent = secondaryEvents[i];
    
    // Create a unique key based solely on the start and end times.
    // (Note: this assumes that no two distinct events share exactly the same times.)
    var key = secEvent.getStartTime().getTime() + "_" + secEvent.getEndTime().getTime();
    
    // Skip if we've already processed an event for this time slot.
    if (processed[key]) {
      Logger.log("Skipping event (already processed): " + key);
      continue;
    }
    
    // If the event is an all-day event or falls on a weekend, skip it.
    if (secEvent.isAllDayEvent()) {
      Logger.log("Skipping all-day event for " + secEvent.getStartTime());
      continue;
    }
    var day = secEvent.getStartTime().getDay();
    if (day < 1 || day > 5) {
      Logger.log("Skipping non-weekday event for " + secEvent.getStartTime());
      continue;
    }
    
    // Check if a matching event already exists in the primary calendar (by time).
    var exists = false;
    for (var j = 0; j < primaryEvents.length; j++) {
      var primEvent = primaryEvents[j];
      // We use start/end times for matching, since titles/details may be unavailable.
      if (
        primEvent.getStartTime().getTime() === secEvent.getStartTime().getTime() &&
        primEvent.getEndTime().getTime() === secEvent.getEndTime().getTime()
      ) {
        exists = true;
        Logger.log("Found matching primary event for time slot: " + key);
        break;
      }
    }
    if (exists) {
      // Mark it as processed so we don't process it again later.
      processed[key] = true;
      continue;
    }
    
    // Create a new event in the primary calendar marked as "Booked".
    try {
      var newEvent = primaryCal.createEvent('Booked', secEvent.getStartTime(), secEvent.getEndTime());
      // Optionally, add a marker in the description to indicate it was synced (for debugging)
      newEvent.setDescription("sync: booked; key: " + key);
      Logger.log("Created event 'Booked' in primary calendar for time slot: " + key);
      // Mark this time slot as processed.
      processed[key] = true;
    } catch (e) {
      Logger.log("Error creating event for time slot " + key + ": " + e);
    }
  }

  // Optionally: Remove primary events that no longer exist in the secondary free/busy data.
  // Since free/busy doesn't supply details, you could loop through primary events marked with your sync marker
  // and delete any whose time slot is not present in the current secondary events.
  for (var i = 0; i < primaryEvents.length; i++) {
    var primEvent = primaryEvents[i];
    // Only consider events that were created by this sync process
    if (primEvent.getDescription().toLowerCase().indexOf("sync: booked") === -1) {
      continue;
    }
    var primKey = primEvent.getStartTime().getTime() + "_" + primEvent.getEndTime().getTime();
    var stillPresent = false;
    for (var j = 0; j < secondaryEvents.length; j++) {
      var secEvent = secondaryEvents[j];
      var secKey = secEvent.getStartTime().getTime() + "_" + secEvent.getEndTime().getTime();
      if (primKey === secKey) {
        stillPresent = true;
        break;
      }
    }
    if (!stillPresent) {
      try {
        primEvent.deleteEvent();
        Logger.log("Deleted primary event 'Booked' for time slot: " + primKey);
        // Also remove it from our processed store.
        delete processed[primKey];
      } catch (e) {
        Logger.log("Error deleting event for time slot " + primKey + ": " + e);
      }
    }
  }

  // Save the updated processed events record
  props.setProperty('processedEvents', JSON.stringify(processed));

  Logger.log("=== Sync Completed ===");
}

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How to Use This Script

1. Open Google Apps Script
   • Go to Google Apps Script
   • Click New Project
2. Paste the Code
   • Replace google-account-1 and 2 with your primary calendar IDs
3. Run the Script
   • Click the Run button
   • Authorize the script when prompted
4. Automate the Sync
   • In Apps Script, go to Triggers (clock icon)
   • Set it to run the function automatically every hour or daily

Final Thoughts: Simplifying Calendar Management

This free and customizable approach eliminates the need for paid tools, giving you full control over how your calendars sync.

While third-party tools offer additional features, this script is an excellent solution for:
✅ Freelancers working with multiple clients
✅ Consultants needing cross-calendar visibility
✅ Business owners managing multiple ventures