Week Number Calculator
Find the ISO-8601 week number for any date — the standard used across Europe and in business calendars.
ISO week number
Day of year
Weekday
How it works
The tool implements the ISO-8601 rule: shift to the nearest Thursday and count weeks from the first Thursday of that year.
Everything is computed locally in your browser — nothing is uploaded.
Frequently asked questions
How is the ISO week defined?
Under ISO-8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. This can place early-January dates in the previous year's last week.