Week Number Calculator (ISO)
ISO week number for any date — and the dates of any week number.
Result
How to use
- 1
Pick a date to see its ISO week number.
- 2
Or enter a year and week number to see that week's Monday-to-Sunday dates.
Questions
What is an ISO week?
Under ISO 8601, weeks start on Monday and week 1 is the week that contains the year's first Thursday (equivalently, the week with 4 January). Most years have 52 weeks; some have 53.
Why can early January be in week 52 or 53?
If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year. Likewise, late December days can fall in week 1 of the next year.
About this Week Number tool
Week Number Calculator (ISO) — ISO week number for any date — and the dates of any week number. It is one of 200 free tools on namasivayam.in, made by software engineer Namasivayam Vadivelan for everyday use in India.
Everything is calculated instantly in your browser: nothing you type or upload is sent to any server, there is no sign-up, and it works on mobile and desktop. Pick a date to see its ISO week number.
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Results are estimates to help you plan. They are not financial, tax or medical advice.