Showing week numbers in your calendar

Outlook

Week numbers are a calendar display option rather than a separate feature. In the desktop app they live in the calendar options, alongside the setting for the first day of the week and the first week of the year — and that second setting is the one that decides whether you get ISO numbering or something else. In Outlook on the web the same options sit under the calendar's appearance settings.

The "first week of the year" choice is worth a moment. Picking the option that describes the first four-day week gives you ISO numbers, the ones the rest of Europe is using. Picking "starts on Jan 1" gives the US-style count instead.

Google Calendar

Google Calendar has no built-in week-number display in the web interface at the time of writing. The common workarounds are a browser extension, or subscribing to a calendar that carries a week-number entry on each Monday. The second approach works everywhere the calendar syncs, including on a phone, which the extension does not.

If you only need the number occasionally, our week number pageanswers it in one glance and needs nothing installed.

iPhone, iPad and Mac

Apple's Calendar has a week-numbers switch in its settings on both iOS and macOS. On a Mac it is in the app's own preferences; on a phone it is under the Calendar entry in the system settings app. Turning it on puts the number at the edge of each week row in month view.

Apple follows the ISO rule and ties the start of the week to your region setting, so a device set to a US region may start weeks on Sunday while still numbering by ISO.

If the number looks wrong

Three things cause almost every disagreement, in this order:

1. The first-day-of-week setting. Monday and Sunday starts put the same date in different weeks for one day out of seven.
2. The first-week-of-year rule. ISO gives week 1 to the week holding the first Thursday; the US convention gives it to whichever week contains 1 January.
3. The turn of the year. Late December and early January legitimately belong to the other year's week under ISO, which looks like a bug and is not.The two conventions comparedshows exactly where they diverge.

If you need a number you can quote to someone in another country without qualifying it, use the ISO one. It depends on the date alone.