Excel Week Number
ISOWEEKNUM
=ISOWEEKNUM(A1) returns the ISO week number for a date in cell A1.
=ISOWEEKNUM(DATE(2026,5,3))WEEKNUM vs ISOWEEKNUM
WEEKNUM can use Sunday-start numbering. ISOWEEKNUM follows ISO Monday-start week rules.
| Formula | Week system | Use when |
|---|---|---|
=ISOWEEKNUM(A1) | ISO, Monday-start | You need ISO 8601 week numbers. |
=WEEKNUM(A1,1) | Sunday-start | You need a US-style week number. |
=WEEKNUM(A1,21) | ISO-style | You are maintaining older workbooks that use WEEKNUM. |
Examples
| Excel formula | Result | Note |
|---|---|---|
=ISOWEEKNUM(DATE(2026,5,3)) | 18 | Sunday, May 3 ends ISO Week 18. |
=ISOWEEKNUM(DATE(2026,12,28)) | 53 | 2026 has an ISO Week 53. |
Common mistakes
Be careful near New Year. The ISO week-year can differ from the date's calendar year.
Excel's ISOWEEKNUM returns only the week number. If you also need the ISO week-year, test dates such as 2025-12-29 and 2027-01-03 with a separate ISO year formula or the WhatWeek calculator.