Week Number Calculator

Find the ISO 8601 week number for any date, or look up which dates fall in a given week number and year. ISO weeks run Monday to Sunday, with Week 1 defined as the week containing January's first Thursday. View the full year week calendar with the current week highlighted.

Result

ISO Week NumberWeek 13
ISO Year2026
Day of WeekSaturday
Week Starts (Mon)2026-03-23
Week Ends (Sun)2026-03-29

Frequently Asked Questions

What is the ISO week number for today?

Use the Week Number Calculator above — it defaults to today's date and instantly shows the current ISO week number, the ISO year, and the Monday-to-Sunday span of the current week. The full year table highlights the current week for quick reference.

What is an ISO week number?

An ISO week number is defined by ISO 8601, the international date and time standard. ISO weeks run Monday to Sunday. Week 1 of a year is the week that contains the year's first Thursday (equivalently, the week containing January 4). A year has 52 or 53 ISO weeks.

How is ISO week 1 determined?

ISO Week 1 is the week that contains the first Thursday of the calendar year. This means January 1 may be in the last week of the previous ISO year if it falls on Friday, Saturday, or Sunday, and similarly December 31 may be in ISO Week 1 of the next year if it falls on Monday, Tuesday, or Wednesday.

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

Because ISO weeks always start on Monday and Week 1 is anchored to the first Thursday, a few days at the start or end of each calendar year may belong to a different ISO week year. For example, December 31, 2019 is in ISO Week 1 of 2020, not week 53 of 2019. The calculator always shows the correct ISO year alongside the week number.

How do ISO week numbers differ from US week numbers?

ISO weeks start on Monday, with Week 1 defined as the week containing January 4 (the first Thursday). US week numbering typically starts on Sunday, and Week 1 is simply the week containing January 1. This means the same date can have a different week number depending on which system is used, especially near the start of the year.

Which years have 53 ISO weeks?

A year has 53 ISO weeks when January 1 falls on Thursday (in any year), or when January 1 falls on Wednesday in a leap year. Recent 53-week years include 2015, 2020, and 2026. The full year week table in the calculator shows all weeks for any selected year, so you can see whether it has 52 or 53 weeks.

How do I find the date for a given week number?

Switch to 'Week Number to Date' mode in the calculator, enter the year and ISO week number, and the calculator shows the Monday start date and Sunday end date of that week. Week 1 of 2026, for example, starts on Monday December 29, 2025 and ends on Sunday January 4, 2026.

How are week numbers used in business and software?

ISO week numbers are used in agile sprint planning (e.g., 'Sprint Week 14'), supply chain scheduling, financial reporting (fiscal weeks), EU labor regulations, and many date/time libraries (Python's isocalendar(), JavaScript's Temporal API, SQL EXTRACT(WEEK ...)). Most European and international systems use ISO weeks; US-based systems often use the Sunday-start convention.

ISO 8601 Week Number Rules

The ISO 8601 standard defines a week numbering system used across Europe, most of Asia, and in many international business contexts. Under ISO 8601:

  • Week 1is the week that contains the year's first Thursday — equivalently, the week containing January 4.
  • Each ISO week runs from Monday to Sunday.
  • A year has either 52 or 53 ISO weeks. A 53rd week occurs when January 1 falls on Thursday, or (in a leap year) on Wednesday or Thursday.
  • The ISO week year may differ from the calendar year for days near January 1 and December 31. For example, December 31, 2019 belongs to ISO week 1 of 2020.

ISO Week Number Formula

  1. Find the Thursday of the week containing the date.
  2. The ISO year is the calendar year of that Thursday.
  3. Week number = ⌈ (days from Jan 1 of ISO year to that Thursday + 1) / 7 ⌉

Week Numbering Differences: US vs ISO

The United States uses a different week numbering convention from ISO 8601. Understanding both systems prevents confusion in international projects and reports.

PropertyISO 8601US (Common)
Week starts onMondaySunday
Week 1 definitionContains Jan 4 (first Thursday)Contains Jan 1
Total weeks per year52 or 5352 or 53 (different dates)
StandardISO 8601 (international)No formal standard
Used inEurope, Asia, international businessUS calendars, some software
Jan 1 always in Week 1?No (may be in prior year's week)Yes

This calculator uses the ISO 8601 standard (Monday start, Week 1 = first Thursday rule), which is what most programming languages and international reporting tools use.

How to Find the Week Number for a Date

To find the ISO week number for any date, use the Date to Week Number mode above:

  1. Select Date to Week Number at the top of the calculator.
  2. Enter the date using the date picker, or click the calendar icon to use today's date.
  3. The result instantly shows the ISO week number, ISO year, day of the week, and the full Monday–Sunday span of that week.

Example: January 1, 2026 is a Thursday — it falls in ISO Week 1 of 2026, which runs Mon Dec 29, 2025 through Sun Jan 4, 2026.

Full Year Week Table

The All Weeks in [Year] table at the bottom of the calculator lists every ISO week of the selected year, showing the Monday start date and Sunday end date for each week. The current week is highlighted for quick reference. This is useful for:

  • Planning sprint schedules and fiscal quarters
  • Verifying which week a specific date falls in
  • Identifying years with 53 ISO weeks (long years)
  • Cross-referencing week numbers between different systems

Years with 53 ISO weeks (recent examples): 2015, 2020, 2026, 2032. These occur when January 1 falls on Thursday, or when it falls on Wednesday in a leap year.

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