Week number
Find the ISO week number for a date.
Choose a date and instantly get its ISO week number. The tool also shows the ISO year and the start and end dates of the matching week.
Date to analyze
Result
ISO week
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- ISO year
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- Week start
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- Week end
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Quick explanation
Select a date in the field. The week number updates automatically. The result shows the ISO week, the ISO year, the Monday starting the week and the Sunday ending it.
Try this too
Need the exact weekday?
Use the day of the week tool to find whether a date falls on a Monday, Tuesday or another day.
Additional information
Find the week number of a date
This tool quickly tells you which ISO week a date belongs to. Just choose a date: the week number updates automatically.
This reference is useful for organizing schedules, tracking work periods, preparing calendars, sorting events or checking deadlines expressed as week numbers.
What the result shows
The main result displays the ISO week number. The details show the ISO year as well as the start and end dates of the matching week.
Showing the beginning and end of the week makes it easy to check the exact period without calculating the Monday and Sunday separately.
How does the ISO week work?
An ISO week starts on Monday and ends on Sunday. This format is used in many calendars, agendas and planning tools.
Week 1 of an ISO year is the week that contains the first Thursday of the year. Another way to say it is that a week belongs to the year of its Thursday. This rule keeps week numbering consistent, even around the turn of the year.
Why the ISO year can be different
Around January 1 and December 31, the ISO year shown by the tool may be different from the calendar year of the selected date. For example, some early January dates may still belong to the last ISO week of the previous year.
Conversely, some late December dates may already belong to week 1 of the next ISO year. This is not an error: it is simply the result of the first-Thursday rule.
Limits and reuse
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