Date & Time
ISO Week Number
Determine the ISO week number (1-52/53) for any given date. ISO weeks start on Monday and the first week contains the first Thursday of the year.
Excel Formula
=ISOWEEKNUM(A2)Step-by-Step Explanation
1
ISOWEEKNUM returns the ISO 8601 week number
2
A2 is the date to evaluate
3
ISO weeks start on Monday
4
Week 1 is the week containing the first Thursday of the year
5
Returns values 1 through 53
Example
| Date (A) | ISO Week (B) |
|---|---|
| 1/5/2026 | 2 |
| 3/18/2026 | 12 |
| 12/31/2026 | 53 |
Result: Formula in B2: =ISOWEEKNUM(A2) → 2
Common Variations
US week number
=WEEKNUM(A2)US system where Week 1 starts Jan 1
Week number + year
=YEAR(A2)&"-W"&TEXT(ISOWEEKNUM(A2),"00")Returns 2026-W12
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