ISO Week Numbers Explained
An ISO week number labels each week of the year from 1 to 52, or 53 in some years, under the international standard ISO 8601. Weeks run Monday to Sunday, and week 1 is the week that contains the year's first Thursday, which is the same as the week containing January 4. This is the week numbering you see down the side of many printable calendars and in payroll, retail, and project planning.
What is an ISO week number?
ISO 8601 is the international date and time standard, and its week-numbering rules make week numbers mean the same thing everywhere. Three rules define the system: a week starts on Monday, week 1 always contains the first Thursday of the year, and every week belongs to the year that holds most of its days. Because of the first-Thursday rule, the opening days of January sometimes fall in the last week of the previous year, and the final days of December sometimes fall in week 1 of the next year.
The week 1 rule (first Thursday, or January 4)
The simplest way to find week 1 is to locate the year's first Thursday: the week that holds it, counting from the Monday before, is week 1. An equivalent test is that week 1 is always the week containing January 4, because January 4 can never fall earlier than that first Thursday's week. So when January 1 is a Friday, Saturday, or Sunday, those days belong to the last week, 52 or 53, of the previous year, and week 1 starts a few days later.
ISO week numbers vs other systems
Not every calendar counts weeks the same way. North American and spreadsheet calendars often start the week on Sunday and define week 1 differently, so the same date can carry a different week number depending on the system.
| Feature | ISO 8601 | North American (common US) |
|---|---|---|
| Week starts on | Monday | Sunday |
| Week 1 is | the week with the first Thursday (contains January 4) | the week containing January 1 |
| Weeks per year | 52, or 53 in some years | up to 53, or 54 in a leap year that starts on Saturday, with short weeks at the start and end |
| Typical use | business, payroll, project planning, most of Europe | US wall calendars and spreadsheets |
How to show week numbers on your printable calendar
On PrintAnyCalendar you can turn on Show week numbers for any year from 2001 to 2051, and the ISO week for each row appears down the side of the grid. Print the whole year or a single month as a free vector PDF.
- This week's number? The week number tool shows the current ISO week instantly.
- A full year to print? Open the calendar generator, enable week numbers, and download.
- Why some years reach week 53? See how many weeks are in a year for the 52 vs 53 rule.
Frequently asked questions
When does the ISO week start?
On Monday. Under ISO 8601 every week runs Monday to Sunday.
What is ISO week 1?
The week containing the year's first Thursday, which is always the week that holds January 4.
How many ISO weeks are in a year?
Usually 52, and 53 in years whose January 1 is a Thursday, or a Wednesday in a leap year. See how many weeks are in a year.
Why is January 1 sometimes in week 52 or 53?
Because week 1 is fixed to the first Thursday. When January 1 falls on a Friday, Saturday, or Sunday, it belongs to the final week of the previous year.
Week numbering follows ISO 8601, in which weeks begin on Monday and week 1 contains the year's first Thursday.