Week Number

How it works

The ISO week is defined by the first Thursday of the year. Those Thursdays anchor week 1, and every week runs Monday to Sunday. Because of that definition, a few days at the start or end of a calendar year can belong to a different ISO week-year.

Examples

• January 1, 2026 is a Thursday, so it is ISO week 1 of 2026.
• December 31, 2025 belongs to ISO week 1 of 2026, because the Thursday of its week is January 1, 2026.
• December 31, 2020 is ISO week 53 of 2020.

Need the weekday a date falls on instead? Try the Day of the Week tool.

Frequently asked questions

Related tools

• Days Between Dates

  Count calendar days between two dates with a clear inclusion choice.

• Business Days Between Dates

  Count weekdays, with an optional observed US federal-holiday policy.

• Age Calculator

  Find age in years, months, and days between a birth date and an as-of date.

• Hours Between Times

  Find an elapsed duration between two 24-hour times.

• Date Calculator

  Add or subtract calendar days, weeks, months, or years.

• Days From Today

  Find the date that is N days, weeks, months, or years from today.

• Business Days From Today

  Find the date that is N business days from today.

• Day of the Week Calculator

  See what day of the week a date was or will be.

• Decimal Hours Calculator

  Convert hours and minutes to decimal hours, or decimal hours back to HH:MM.

• Weeks Between Dates

  Count the weeks and days between two calendar dates.

• Minutes Between Times

  Find the elapsed minutes between two 24-hour times.

• Work Hours Calculator

  Calculate worked hours between two times with an unpaid break.

• Add or Subtract Business Days

  Move a date forward or backward by a number of business days.

• Time Zone Converter

  Convert a time between named time zones, resolving daylight-saving changes honestly.

• Days Until

  Count the whole calendar days from today to a chosen date.

• Leap Year Checker

  Check whether a year is a leap year on the Gregorian calendar.

• Add or Subtract Time

  Shift a time of day by an amount of hours and minutes.

• Date & Time Duration

  Measure the span between two full dates and times, in days, hours, and minutes.

• Next Weekday

  Find the date of the next or previous occurrence of a chosen weekday.

• Weekends Counter

  Count how many weekend days fall between two calendar dates.

• Time Unit Converter

  Convert between hours, minutes, seconds, days and weeks.