Syntax
ISOWEEKNUM(date)
| Parameter | Description |
|---|---|
| date | Parameter of the ISOWEEKNUM function. |
Examples
ISO week number
Formula
=ISOWEEKNUM("2026-01-01")
Returns 1 — January 1, 2026 is a Thursday, so it falls in ISO week 1 of 2026.
Year-end edge case
Formula
=ISOWEEKNUM("2025-12-31")
Returns 1 — December 31, 2025 is a Wednesday that belongs to ISO week 1 of 2026, not week 53 of 2025.
Weekly reporting label
Formula
="W" & TEXT(ISOWEEKNUM(A1), "00")
Creates labels like "W01", "W15", "W52" with leading zeros for consistent sorting in reports.
Common Errors
#VALUE!
Occurs when the argument is not a valid date.
#NUM!
Occurs for date serial numbers outside the valid range.
Tips
ISO vs WEEKNUM
ISOWEEKNUM always uses Monday as week start and the first-Thursday rule. WEEKNUM is locale-flexible. Use ISOWEEKNUM for ISO 8601 compliance.
Year boundary behavior
A date in late December may be ISO week 1 of the NEXT year, and early January dates may be ISO week 52/53 of the PREVIOUS year. Pair with a year function carefully.
ISO year calculation
For the ISO year (which may differ at year boundaries): =YEAR(A1-WEEKDAY(A1,2)+4). This returns the ISO year that the week belongs to.
Try ISOWEEKNUM in Viztab
Import your data and use ISOWEEKNUM with 370+ other formulas. No signup required.
Open Viztab