Week Number

Find the ISO week number for any date. Based on the ISO 8601 standard, this calculator helps with project planning, scheduling, and calendar management. Free online week number tool.

Select a date to find the ISO 8601 week number

How to Use

  1. Select a date

    Choose the date for which you want to find the week number.

  2. Run the calculation

    Click Calculate to determine the ISO week number.

  3. View the result

    See the week number along with the start and end dates of that week.

What is an ISO 8601 week number?

The ISO 8601 week number is an international standard that labels each week of the year with a sequential number from week 1 to week 52 (or 53). Two rules sit at its core. First, a week starts on Monday and ends on Sunday. Second, week 1 is the week that contains the year's first Thursday, which is the same as saying 'the week containing January 4 is always week 1'.

Why Thursday is the anchor

Thursday decides which year a week mostly belongs to. If more than half of a week falls in the new year, it becomes week 1; otherwise it stays as the last week of the previous year. This keeps weeks from being split ambiguously across year and month boundaries.

That consistency is exactly why manufacturing schedules, accounting close, weekly retail and logistics reports, and global project management (for example 'launch in week 24') use the week number as a shared unit instead of a raw date.

Calculation Formula

Shift the input date to the Thursday of its week, then derive the week number from how many days have passed since January 1 of the year that Thursday falls in.

weekday (Mon=1...Sun=7) -> d
Thursday = date + (4 - d) days
week number = ceil(day of year of Thursday / 7)

Example: 2026-01-01 (Thu)

  • weekday d = 4 (Thu), adjustment 4 - 4 = 0 -> Thursday stays 2026-01-01
  • day of year = day 1
  • week number = ceil(1 / 7) = week 1, ISO year 2026

By contrast, 2027-01-01 (Fri) moves its Thursday to 2026-12-31, so it is counted as week 53 of 2026.

Frequently Asked Questions

How is the week number determined?
Following the ISO 8601 international standard, Monday is the first day of the week, and week 1 is the week that contains the first Thursday of the year.
What is an ISO week number?
The ISO 8601 week number is the week numbering system defined by the international standard. Week 1 is the week containing the year's first Thursday, which means the week with January 4 is always week 1. It is widely used for weekly scheduling in business, manufacturing, and IT, and a year has either 52 or 53 weeks.
Is January 1 always in week 1?
No. Under ISO 8601, if January 1 falls on a Friday, Saturday, or Sunday, that date may belong to the last week (52 or 53) of the previous year. Conversely, December 29-31 can fall in week 1 of the next year. This is why the ISO week year can differ from the calendar year.
How many weeks are in this year?
Most years have 52 weeks, but a year has 53 weeks when January 1 is a Thursday, or when it is a leap year whose January 1 is a Wednesday. The result also shows the total number of weeks for the selected year.
Does the US use a different week numbering method?
Yes. In the United States and Canada the week is often counted as starting on Sunday, with the week containing January 1 treated as week 1. This calculator uses the international standard (ISO 8601): weeks start on Monday and week 1 is the one containing the first Thursday.
Do week numbering rules vary by country?
Yes. ISO 8601 (the European and international standard) starts weeks on Monday and counts the week with the first Thursday as week 1, while the US and Canada commonly start the week on Sunday and count the week containing January 1 as week 1. The same date can land in a week that differs by one depending on the rule.
Why show the day of year and leap year status together?
The day of year tells you how many days into the year the chosen date is, counting from January 1, which is handy for progress tracking and pro rata calculations. In a leap year February has 29 days, so the year has 366 days, meaning the same date's day of year and remaining days change depending on whether it is a leap year.
2026 calendar data

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