Free Week Number Calculator
ISO week number for any date.
Date → ISO week number
W43
ISO week
2026
Week year
Fri, October 23, 2026
Full date
Q4
Quarter
296
Day of year
ISO week → dates
ISO week 1 contains the first Thursday of the year.
Week 43 of 2026
Monday Mon, October 19, 2026 → Sunday Sun, October 25, 2026
ISO-8601 weeks start on Monday. Week 1 is the week containing the year’s first Thursday — it can begin on December 29 of the previous year.
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How to use the Week Number Calculator
- 1Pick a date.
- 2See its week number.
- 3Or enter a week to see its dates.
Features
- ISO-8601
- Both directions
- Quarter display
Frequently Asked Questions
Why does week 1 sometimes start in December?
ISO week 1 is the week containing the first Thursday of the year — it can begin Dec 29.
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About the Week Number Calculator
Find the ISO-8601 week number of any date and which dates a given week covers. The Week Number Calculator runs entirely in your browser — your data is processed locally and never uploaded to a server. Explore more Everyday tools or browse all free online tools.