DailyNumeric
Age, Date & Calendar

Week Number Calculator

Enter a date to find its ISO 8601 week number.

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Any past or future date works — try 2026-10-28 for today.

Enter a date to find its ISO week number.

How it works

The calculator uses the ISO 8601 standard, where weeks run Monday to Sunday and week 1 of a year is defined as the week containing that year's first Thursday.

Formula & methodology

This is the ISO 8601 week number. Weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday. Because of this, a few days at the very start or end of a year can belong to a neighbouring year's week numbering.

Step 1Thursday = Date + (4 − ISO weekday)
Step 2Week number = ⌊ (Thursday − 1 January of Thursday's year) ÷ 7 ⌋ + 1
Week-yearISO week-year = the calendar year of that Thursday

What each term means

ISO weekday
Monday = 1 … Sunday = 7.
Thursday
The Thursday in the same Monday-to-Sunday week as your date.

Worked example

January 1, 2027 falls in ISO week 53 of 2026, since it belongs to the Monday–Sunday week that started in late December.

Common uses

  • —Filling in timesheets, invoices, or project plans that reference ISO week numbers instead of dates.
  • —Coordinating with European businesses and systems where week numbers are commonly used for scheduling.
  • —Checking which week number a specific date falls in for reporting periods.

Important notes

  • —Because ISO weeks start on Monday and are anchored to the first Thursday of the year, dates in very early January or very late December can belong to the week-year before or after the calendar year — this is expected and is called out in the result.
  • —Most years have 52 ISO weeks; some have 53, which the result shows automatically.