The formula
ISO week (fill down)ExcelGoogle Sheets
=ISOWEEKNUM(A2)In C2 → 1
WEEKNUM (week starts Monday, system 1) (fill down)ExcelGoogle Sheets
=WEEKNUM(A2,2)In D2 → 1
ISO year-week label (fill down)ExcelGoogle Sheets
=YEAR(A2-WEEKDAY(A2,2)+4)&"-W"&TEXT(ISOWEEKNUM(A2),"00")In E2 → 2026-W01
✓ Tested. All 9 Excel results in this example were recalculated in LibreOffice Calc 25.2 and matched the values shown.
Worked example
Paste the data below into a blank sheet starting at cell A1 (or download the cheat sheet, where every example is set up). Highlighted cells hold the formulas and show the expected results.
| A | B | C | D | E | |
|---|---|---|---|---|---|
| 1 | Date | ||||
| 2 | Jan 1, 2026 | 1 | 1 | 2026-W01 | |
| 3 | Jan 1, 2027 | 53 | 1 | 2026-W53 | |
| 4 | Jun 15, 2026 | 25 | 25 | 2026-W25 |
How it works
- ISOWEEKNUM(A2)
- ISO 8601 week number (1–53).
- WEEKNUM(A2,2)
- Week 1 always contains January 1, weeks start Monday.
- A2-WEEKDAY(A2,2)+4
- The Thursday of that week; its year is the ISO year.
Common errors and fixes
Jan 1 shows week 53?
Correct in ISO: Jan 1, 2027 belongs to ISO week 53 of 2026. Use the ISO year label so it isn't confused with 2027.
WEEKNUM and ISOWEEKNUM disagree?
They use different rules around New Year. Pick one and use it everywhere.
Why does the formula return #NAME??
Excel 2010 or older has no ISOWEEKNUM; use WEEKNUM(A2,21).
Does this work in Google Sheets?
Yes. The formulas on this page work the same way in Google Sheets.