Why does January 1 sometimes belong to the previous year's week?

In ISO 8601, if January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year because the majority of that week's days are in the old year. This happens about 3 out of every 7 years.

Can a year have 53 weeks?

Yes. An ISO year has 53 weeks if January 1 is a Thursday, or if December 31 is a Thursday. This occurs roughly every 5-6 years. Examples: 2004, 2009, 2015, 2020, and 2026 all have 53 ISO weeks.

What is the difference between ISO and US week numbering?

ISO weeks start on Monday and week 1 contains January 4. US weeks often start on Sunday and week 1 contains January 1. The same date can have different week numbers in each system, especially in early January and late December.

Why are week numbers useful?

Week numbers provide a compact reference for business planning: 'deliver by W12' is unambiguous. They simplify scheduling recurring events, comparing year-over-year data, and coordinating across time zones where date formats differ.

How do week numbers relate to fiscal years?

Many retail and manufacturing companies use fiscal calendars based on weeks (the 4-4-5, 4-5-4, or 5-4-4 systems) to ensure equal comparison periods. Each fiscal month has either 4 or 5 full weeks, totaling 52 or 53 weeks per fiscal year.

Week Number Calculator

Find the ISO week number for any date — and the dates in any week

Week Number Calculator

Find the ISO week number for any date

Week Number

Enter a date

Date (YYYY-MM-DD)

Formula

Week = ceil(day of year / 7)

What Is an ISO Week Number?

ISO week numbers divide the year into 52 or 53 numbered weeks, each starting on Monday and ending on Sunday. Week 1 is defined as the week containing the year's first Thursday. This system is standardized by ISO 8601 and is widely used in European business, project management (Gantt charts), payroll cycles, and manufacturing scheduling. Knowing the week number makes it easy to reference a specific period without naming exact dates.

This calculator tells you the ISO week number for any date you enter, the week year it belongs to (which can differ from the calendar year for dates near January 1 or December 31), and the full Monday-to-Sunday date range of that week. You can also go the other way — enter a year and week number to see exactly which dates that week covers.

How to Use the Week Number Calculator

Enter any date using the date picker or type it in the input field.
Click Calculate to run the lookup.
See the ISO week number, the week year it belongs to, and the full date range (Monday through Sunday) for that week.
Alternatively, enter a year and a week number to find the corresponding Monday-to-Sunday date range.

ISO Week Number Formula & Rules

ISO Week 1 = the week containing the first Thursday of the year

Equivalent rules (all produce the same result):
  1. The week containing January 4
  2. The first week with a majority of days (4 or more) in the new year
  3. The week of the year's first Thursday

Weeks per year:
  Most years: 52 weeks
  Long years:  53 weeks (when Jan 1 or Dec 31 falls on Thursday)

Week year != calendar year near Jan 1 / Dec 31:
  Jan 1, 2023  -> Week 52 of 2022  (it was a Sunday)
  Dec 31, 2020 -> Week 53 of 2020  (it was a Thursday)

ISO weeks always start on Monday and end on Sunday. The "week year" (written as 2022-W52) may be one year earlier or later than the calendar year for dates that fall in early January or late December.

Examples

January 1, 2024 (Monday)

Because January 1, 2024 is a Monday and that Monday falls in the first week containing a Thursday (January 4), this date is Week 1 of 2024. The week runs January 1–7, 2024.

December 31, 2023 (Sunday)

December 31, 2023 is a Sunday. The week it belongs to started on Monday, December 25, 2023. That week's Thursday is December 28 — still in 2023 — so this is Week 52 of 2023, running December 25, 2023 through December 31, 2023.

March 15, 2024

March 15, 2024 falls in ISO Week 11 of 2024. The week runs from Monday, March 11 through Sunday, March 17, 2024.

Frequently Asked Questions

Why does week 1 sometimes start in December of the previous year?▾

Under ISO 8601, Week 1 is defined as the week containing the first Thursday of the year. If January 1 falls on a Friday, Saturday, or Sunday, then the first Thursday of the year is actually in the following week — meaning the days from January 1 to that first Thursday are assigned to Week 52 or 53 of the previous year. For example, January 1, 2021 was a Friday, so it belonged to Week 53 of 2020.

How many weeks are in a year?▾

Most years have 52 ISO weeks. A year has 53 weeks — called a long year — when January 1 or December 31 falls on a Thursday. This happens roughly 71 times per 400-year cycle. Recent long years include 2015, 2020, and 2026.

What is ISO 8601?▾

ISO 8601 is an international standard published by the International Organization for Standardization (ISO) that defines a consistent way to represent dates, times, and durations. Its week numbering system ensures that every week is whole (Monday–Sunday) and unambiguously assigned to a single week year, making it ideal for business reporting and scheduling across countries.

How are week numbers used in business?▾

Week numbers are a standard shorthand in European business, logistics, manufacturing, and project management. Teams say things like "ship in W34" or "the sprint closes in W22" instead of quoting specific dates. Gantt charts, production schedules, and payroll systems in countries like Germany, Sweden, and the Netherlands are routinely organized by week number.

Does the US use the same week numbering system?▾

Not always. The US commonly uses a different convention where weeks start on Sunday and Week 1 is simply the week containing January 1 — regardless of which day that falls on. This can produce different week numbers than ISO 8601. Software applications in the US (including some spreadsheet functions) may default to the US convention, so it is worth confirming which system you are using when sharing week numbers internationally.