Week numbers in Excel, Google Sheets and Outlook

In Excel 2013 and later, the shortest way to get the ISO week number from a date in cell A2 is =ISOWEEKNUM(A2). If you are on an older version, or want to be explicit about the numbering system, use =WEEKNUM(A2,21) — return type 21 is Excel's code for the ISO-8601 system.

Be careful with the more familiar =WEEKNUM(A2,2). It starts weeks on Monday, which looks right, but it does not follow the ISO rule for week 1, so the result can be off by one around the turn of the year. If the number has to match a project plan, a payroll period or anything else international, use the ISO formula.

Google Sheets has the same function: =ISOWEEKNUM(A2) returns the ISO week number for a date, and =WEEKNUM(A2,21) does exactly the same thing. Both treat weeks as Monday-to-Sunday, so the number they produce matches this site.

To pull the week number out of a date you typed as text, wrap it in DATEVALUE first, for example =ISOWEEKNUM(DATEVALUE(A2)).

Outlook can label the weeks in its calendar view. On Windows, open File → Options → Calendar and tick Show week numbers in the month view and the Date Navigator. Outlook then prints the week number down the left-hand side of the month grid, which makes it easy to check a date without leaving your mail.

A week number is only meaningful together with the system it comes from. ISO-8601 counts weeks from Monday, and week 1 is the week containing the first Thursday of January. The US convention counts weeks from Sunday and starts week 1 on January 1. In the first days of January, and sometimes at the end of December, those two systems give different answers for the same day.

Tools inherit whatever their default is, so a number copied from one program into another can look wrong even though neither is broken. When you compare week numbers across a team, agree on ISO-8601 — that is what the calendars on this site use.

ISO-8601 also defines a way to write a full date as a week number: 2026-W15-3. The year comes first, then W and the week number, then the day of the week where 1 is Monday and 7 is Sunday. That format is common in manufacturing, logistics and file naming because it sorts correctly and never depends on local date conventions.

If you want to verify what a formula should return, look up the date below — the calculator uses the same ISO-8601 rules as the Excel formula above.

Calendar weeks 2026Calendar weeks 2027Calendar weeks 2028How ISO week numbers work