ISO Week Number Calculator — date to week & back
Find the ISO-8601 week number, ISO year and weekday for any date, see the current week, and look up the Mon–Sun dates for a given year and week.
🔒 Runs entirely in your browser. Your input and output are never sent to a server or stored — fully private.
ISO-8601 weeks start on Monday. Week 1 is the week containing the first Thursday of the year (it always contains 4 January).
Find a week’s dates
About this tool
Enter any date and this tool gives you its ISO-8601 week number along with the matching ISO year, and it works in reverse too: type a year and a week number to get that week's Monday-to-Sunday date range. ISO weeks always start on Monday, which makes them the standard for business calendars, payroll and project planning. The lookup runs entirely in your browser, so your dates stay on your device.
Frequently asked questions
How is the ISO week number defined?
Under ISO-8601 every week starts on Monday, and week 1 is the week that contains the year's first Thursday — equivalently, the week holding 4 January. That rule keeps each week whole, so a year has 52 or 53 full weeks and no stray partial weeks at either end.
Why does the ISO year sometimes differ from the calendar year?
Because ISO weeks never split, the first few days of January can belong to the last week of the previous year, and the last days of December can fall in week 1 of the next year. For example, 1 January 2023 was a Sunday, so it sat in week 52 of ISO year 2022. That is why the tool shows the ISO year as well as the week number.
Is the lookup done on my device?
Yes. Both the date-to-week and the week-to-date lookups are calculated entirely in your browser, so the dates and week numbers you enter never leave your device or get sent to a server. This makes it safe for internal project schedules and quick enough to give an answer the instant you type.