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Week Number

Find the ISO week number and week year for any date, with the Monday to Sunday week range.

Week number for a date

ISO week: —
Week year: —
Week runs from — to —
Weekday: — · Day of the year: —
Days left in the year: — · Quarter: —
ISO weeks start on Monday, and week 1 is the week that contains the first Thursday of the year.

How to use the Week Number

  1. Pick a date with the date selector, or press Today.
  2. The ISO week number and its week year are shown at once.
  3. The Monday to Sunday range and the day of the year are listed below.

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Frequently Asked Questions

Common questions about the free online week number.

It is the number of the week within a year under the ISO 8601 standard, which is used for business and academic calendars across Europe and much of.
ISO weeks always start on Monday and end on Sunday, unlike the Sunday-first convention used in some countries.
Around New Year a few days belong to the next or previous week year. For example 1 January 2027 falls in week 53 of 2026, so the week year is 2026.
Week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January.
A year has 52 weeks plus one day, or 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year.
It is the position of the date within its calendar year, so 1 January is day 1 and 31 December is day 365 or 366 in a leap year.
Yes. Any date the picker accepts, from historical years to far future years, is calculated the same way.
Enter the Monday of that week if you know it, or try dates around the start of the year until the week number matches.
No. The week number is calculated in your browser from the date you choose.

Last updated: October 10, 2026