Week Number Calculator
Find the ISO 8601 week number for any date.
ISO week number
Week 37
2026
How it works
Follows the ISO 8601 standard, where weeks start on Monday and week 1 is the week containing the year's first Thursday.
Why does the week number sometimes belong to a different year?
The last days of December or first days of January can fall in week 1 of the next year or the last week of the previous year under ISO rules.