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Week Number Calculator

ISO week number, day of year, and days remaining — for any date.

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A Small Utility, Genuinely Used

Week-numbered planning shows up more than people expect — supply chain schedules, manufacturing batch tracking, and agricultural planning all commonly reference an ISO week number rather than a calendar date. This tool computes it correctly, along with the day-of-year count and how much of the year remains.

How to Use This Tool

  1. 1Pick a dateDefaults to today.
  2. 2Read the ISO week numberStandard week numbering used in business and manufacturing planning.
  3. 3Check the day-of-year countHow many days into the year this date falls.
  4. 4See days and weeks remainingA quick sense of how much of the year is left.

Good to Know

  • ISO week numbering starts weeks on Monday, and week 1 is defined as the week containing the year's first Thursday — which is why a date in late December or early January can sometimes belong to a different "ISO year" than the calendar year.
  • Week-numbered planning is genuinely common in supply chain, manufacturing, and agricultural scheduling contexts, not just a trivia fact.
  • Day-of-year (sometimes called "Julian date" in some industries, though that term means something different in astronomy) is used in various inventory and batch-tracking systems.

Frequently Asked Questions

Why does the ISO week number sometimes differ from the calendar year?

Because ISO week 1 is defined by containing the year's first Thursday — a date like December 31st can fall in "week 1" of the following ISO year, or January 1st can fall in the last week of the previous ISO year, depending on which day of the week it lands on.

What is day-of-year used for?

It's a simple running count of days since January 1st, used in various planning, inventory, and scheduling systems where a plain calendar date is less convenient than a sequential day count.

Does this account for leap years?

Yes — days remaining in the year and day-of-year both correctly account for the extra day in a leap year.