Get the Week Number of a Date

Excel Formulas › Date & Time

All versionsISOWEEKNUMWEEKNUM

To label dates by week number — for weekly reports or sprint planning — use ISOWEEKNUM (the international standard, weeks start Monday) or WEEKNUM (US style, configurable). They can disagree, so pick deliberately.

Quick formula: for a date in A2, the ISO week number:
=ISOWEEKNUM(A2)
ISO weeks start on Monday and week 1 is the week containing the year’s first Thursday — the global business standard.

Functions used (tap for the full reference guide):

ISOWEEKNUMWEEKNUM

The example

The same dates by ISO week.

AB
1DateISO week
21/1/20261
36/17/202625
412/31/202653

The formula

The ISO week number in B2:

=ISOWEEKNUM(A2) // Jun 17, 2026 → week 25

How it works

ISOWEEKNUM follows the international rule:

  1. It returns 1–53. Weeks start on Monday.
  2. Week 1 is the week that contains the year’s first Thursday — so early-January dates can belong to week 52/53 of the previous year, and late-December dates to week 1 of the next.
  3. It needs no configuration — it’s the same everywhere, which is why reports prefer it.
  4. For the US convention (week 1 = the week with Jan 1, weeks start Sunday), use WEEKNUM instead.

Group a report by week: combine the year and ISO week for a unique key — =ISOWEEKNUM(A2) & "/" & YEAR(A2) — or pair with a SUMIFS to total by week.

Try it: interactive demo

Live demo

Pick a date; see its ISO and US week numbers.

ISO week:   US week:

Variations

US week number

Weeks start Sunday, week 1 has Jan 1:

=WEEKNUM(A2)

US weeks starting Monday

Second argument sets the start day (2 = Monday):

=WEEKNUM(A2, 2)

Year + week key

For grouping across years:

=YEAR(A2) & "-W" & TEXT(ISOWEEKNUM(A2), "00")

Pitfalls & errors

ISO and US week numbers differ. ISO starts Monday with a first-Thursday rule; WEEKNUM defaults to Sunday with Jan 1 in week 1. Don’t mix them in one report.

Year boundaries are tricky. Jan 1 can be ISO week 52/53 of the prior year. Pair the week with the correct ISO year if you need a true key — the ISO year can differ from YEAR() at the edges.

ISOWEEKNUM needs Excel 2013+. In Excel 2010 use =WEEKNUM(A2, 21), which applies the ISO rule.

Practice workbook

📊
Download the free Get the Week Number of a Date practice workbook
Dates with live ISOWEEKNUM and WEEKNUM, the Monday-start and year-week-key variants, plus 4 challenges with answers. No sign-up required.

Frequently asked questions

How do I get the week number of a date in Excel?
Use =ISOWEEKNUM(A2) for the international standard (weeks start Monday, week 1 contains the first Thursday), or =WEEKNUM(A2) for the US convention (weeks start Sunday, week 1 contains Jan 1).
What's the difference between ISOWEEKNUM and WEEKNUM?
ISOWEEKNUM follows the ISO 8601 rule used in most business reporting. WEEKNUM defaults to Sunday-start weeks with January 1 in week 1, and its second argument can change the start day.
How do I group data by week?
Create a key like =YEAR(A2)&"-W"&TEXT(ISOWEEKNUM(A2),"00") and total with SUMIFS, or use a PivotTable grouped by week.

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Related formulas: Get the quarter from a date · Sum by month · Days in a month

Function references: ISOWEEKNUM · WEEKNUM