Google Calendar
Web: click the gear icon → Settings → General → View options → tick Show week numbers. Numbers appear at the left of month view.
Important: Google numbers weeks according to your Start of the week setting (same Settings page). For ISO numbers, set the week to start on Monday; with Sunday starts, the numbering follows the US convention and can differ from ISO by one.
Mobile app: the Google Calendar apps for Android and iPhone do not display week numbers. The usual workaround is a subscribed "week numbers" calendar that shows the number as a weekly all-day event, or simply keeping weekyear.com a tap away.
Outlook
Classic desktop (Windows): File → Options → Calendar → Display options → tick Show week numbers in the month view and in the Date Navigator. Then, under Work time, set First day of week to Monday and First week of year to First 4-day week — that combination is exactly ISO 8601.
Outlook on the web / new Outlook: gear icon → Calendar → View → enable Show week numbers; set First day of the week to Monday and First week of the year to First 4-day week.
Mac desktop: Outlook → Settings → Calendar → tick Show week numbers.
Apple Calendar
Mac: Calendar → Settings (⌘,) → Advanced → tick Show week numbers. Apple's numbering follows ISO 8601 when the week starts on Monday (System Settings → General → Language & Region → First day of week).
iPhone / iPad: the built-in Calendar app has no week-number option. Practical workarounds, in order of popularity:
- Subscribe to an ISO week-number calendar feed (search "week numbers ICS") so each week shows an all-day banner like "W29".
- Use a third-party calendar app that supports week numbers (Fantastical, Calendars 5, and others).
- Add a home-screen widget from a week-number app, or bookmark the live week number.
Samsung / Android
Samsung Calendar: menu (☰) → gear icon → tick Show week numbers; the first day of the week is set just above it — choose Monday for ISO numbering.
Other Android calendars: Business Calendar, aCalendar and most serious third-party apps have a week-number toggle in display settings. The stock Google Calendar app, as noted, does not.
Making the numbers match ISO
Seeing week numbers is half the job; seeing the same numbers as your colleagues is the other half. The magic combination in any app is:
First day of week: Monday + Week 1 rule: "first 4-day week" (or "week containing January 4"). That reproduces ISO 8601 exactly.
Apps configured with Sunday starts or a "week containing January 1" rule will disagree with ISO in some years — the mechanics are in week start conventions. To sanity-check any app, compare it against the live ISO week number or the 2026 chart.