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ISO 8601 compliant

Week Number

ISO week number, any date

In short

The ISO week number tells you which week of the year a date falls in, from 1 to 53, with weeks starting on Monday. Enter any date, or open the page for today, and this calculator returns its ISO-8601 week number, the week's Monday-to-Sunday range and the day of the year.

Week Number

What week is it?

What week number is it?

The week number calculator shows the ISO-8601 week number for any date, plus the date range of that week, the day of the year, and how many weeks are left in the year. Open the page and today's week is shown straight away; enter any other date to look it up.

ISO week numbers run from 1 to 53, start each week on Monday, and define week 1 as the week containing the year's first Thursday. That's why a date in early January can belong to week 52 or 53 of the previous year, and a late-December date can fall in week 1 of the next.

Why it matters: Payroll, manufacturing, retail planning and project schedules across Europe and beyond are often organised by ISO week number rather than calendar date.

Week number vs. calendar week

Some calendars number weeks from 1 January regardless of weekday, but the ISO standard is the one used by most business and scheduling software. This tool uses ISO throughout so your week numbers line up with Outlook, Excel's ISOWEEKNUM and enterprise planning systems.

Related tools

To find which weekday a date is, use the day of the week calculator; for the gap between two dates, the date difference calculator; and to count business days, the working days calculator.

FAQ

Common questions

What week number is it right now?
Open the calculator and today's ISO week number is shown automatically, along with the Monday–Sunday date range for the current week.
How are ISO week numbers defined?
ISO-8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday. Weeks are numbered 1 to 53.
Why is 1 January sometimes in week 52 or 53?
Because ISO week 1 must contain the first Thursday of the year, the first few days of January can still belong to the last week of the previous year.
Does this match Excel and Outlook?
Yes. It uses the ISO-8601 standard, the same as Excel's ISOWEEKNUM function and the week numbers shown in Outlook and most business software. To find the weekday too, use the day of the week calculator.
Sources & standards
  • Standard: ISO 8601 week-date system — weeks begin on Monday and week 1 contains the year’s first Thursday.
  • Compatibility: matches Excel’s ISOWEEKNUM and the week numbers shown in Outlook.
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