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.
WEEKNUM(A1,2) starts weeks on Monday but is not ISO numbering: it still puts January 1 in Week 1. Use ISOWEEKNUM(A1) or WEEKNUM(A1,21) for ISO weeks.
| 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. |
Get the ISO week-year, not just the week number
For a real Excel date in A2, this formula returns the ISO week-year. It moves the date to the Thursday in the same week, whose year defines the ISO year.
=YEAR(A2-WEEKDAY(A2,2)+4)Combine that year with a zero-padded week number to produce a label such as 2026-W53:
=YEAR(A2-WEEKDAY(A2,2)+4)&"-W"&TEXT(ISOWEEKNUM(A2),"00")For January 1, 2027, the correct label is 2026-W53. Combining YEAR(A2) with ISOWEEKNUM(A2) would incorrectly produce 2027-W53.
Use =DATE(2027,1,1) to enter that example without ambiguous text parsing. These formulas use English function names and commas; localized Excel may use translated names or semicolon separators.
New Year and leap-day verification table
Check both columns: a correct week number paired with the wrong year is still an incorrect ISO week label.
| Date in A2 | ISOWEEKNUM(A2) | ISO week-year | Combined label |
|---|---|---|---|
2024-02-29 | 9 | 2024 | 2024-W09 |
2025-12-29 | 1 | 2026 | 2026-W01 |
2026-12-28 | 53 | 2026 | 2026-W53 |
2027-01-01 | 53 | 2026 | 2026-W53 |
2027-01-03 | 53 | 2026 | 2026-W53 |
2027-01-04 | 1 | 2027 | 2027-W01 |
Download all 17 ISO boundary examples (CSV).
The file contains expected values, not executable formulas. Import the date column as dates or convert it with DATE before applying the formulas; do not rely on a locale guessing the text format.
January 1–3 belong to 2026 Week 53; the 2027 calendar starts its first ISO week on January 4.
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. Keep the ISO week-year alongside it, and do not interpret an ISO week label as a payroll or fiscal calendar period.
Open the week number calculator
Function references: Microsoft ISOWEEKNUM and Microsoft WEEKNUM.