ReferenceFunctionsDate & Time Functions
ISOWEEKNUM
ISOWEEKNUM: Returns the ISO 8601 week number (`1` to `53`) for a date serial.
Summary
Returns the ISO 8601 week number (1 to 53) for a date serial.
Remarks
- Weeks start on Monday and week 1 is the week containing the first Thursday of the year.
- Input serials are truncated to whole dates before evaluation.
- Serials are read using the Excel 1900 date mapping.
Examples
ISO week at year start
Grid
| Cell | Value | |
|---|---|---|
| No inputs on Sheet1. | ||
Formula
=
Result
Not evaluated yet.
Expected
1
ISO week crossing year boundary
Grid
| Cell | Value | |
|---|---|---|
| No inputs on Sheet1. | ||
Formula
=
Result
Not evaluated yet.
Expected
53
Related functions
FAQ
Can early-January dates return week 52 or 53?
Yes. ISO week numbering can place dates near New Year in the prior ISO week-year.
Runtime metadata
Category
Date & Time
Signature
ISOWEEKNUM(arg1: Number)Arity
min 1, max 1
Arguments
arg1Number · Scalar · coercion NumberLenientText
Caps
PURE
Source