ISO 8601 standard
ISO week numbers explained
How ISO 8601 numbers the weeks of the year, why week 1 starts where it does, and how it differs from the US system.
An ISO week number is a 1 to 53 label for each week of the year.
Under the ISO 8601 standard, every week starts on Monday and ends on Sunday. Week 1 is the week that contains the year's first Thursday, so the numbering stays tied to whole weeks rather than calendar months. Right now it's week 22 of 2026.
Most years run 52 weeks, some run 53. You can browse every week on the full 2026 calendar, convert a specific date with the week calculator, or read on for how the system works.
How ISO week numbering works
ISO 8601 fixes three simple rules. Together they make week numbers consistent across every country and system that follows the standard.
Weeks start on Monday
Every ISO week runs Monday through Sunday. There is no ambiguity about where a week begins.
Week 1 holds the first Thursday
The first week of the year is the one containing the year's first Thursday. An easy shortcut: it's the week that contains January 4.
Years have 52 or 53 weeks
Because weeks are whole units, a year carries either 52 or 53 of them. The 53rd appears when January 1 lands on a Thursday, or a Wednesday in a leap year.
ISO vs US week numbers
The two systems agree for most of the year. They drift apart by a week near the start and end, because ISO ties week 1 to the first Thursday while the US ties it to January 1.
Why early January can belong to last year
This is the part that trips people up. Since week 1 must contain the first Thursday, the opening days of January sometimes fall into the final week of the previous year.
Take 2026. Week 1 starts on December 29, 2025, which is still in 2025. So a date like January 1 can read as week 53 of 2025 under ISO rules, even though the calendar already says the new year. The week, not the month, is the unit that counts.
If you ever see an early-January date land on week 52 or 53, this is why. It's working exactly as ISO 8601 intends.
Which years have 53 weeks
A 53-week year happens when January 1 is a Thursday, or a Wednesday in a leap year. It's the calendar's way of catching up so weeks stay whole. The next 53-week years are:
Linked years have a full calendar ready. The greyed years are still 53-week years, their calendars open as the date gets closer.
For the deeper math on this, see the weeks in a year guide.
Get the ISO week number in Excel, Sheets and code
Here are the formulas for the ISO week number. They work the same in Excel and Google Sheets.
Excel & Google Sheets
=ISOWEEKNUM(TODAY()) ' ISO week of today
=ISOWEEKNUM(A1) ' ISO week of a date in A1
=WEEKNUM(TODAY(), 21) ' alternative ISO modeJavaScript
function isoWeek(d) {
const date = new Date(d);
date.setHours(0, 0, 0, 0);
date.setDate(date.getDate() + 3 - ((date.getDay() + 6) % 7));
const week1 = new Date(date.getFullYear(), 0, 4);
return 1 + Math.round(
((date - week1) / 86400000 - 3 + ((week1.getDay() + 6) % 7)) / 7
);
}
isoWeek(new Date()); // => 22ISO week numbers: questions and answers
What is an ISO week number?
An ISO week number is a label from 1 to 52 (or 53) that identifies each week of the year under the ISO 8601 standard. Every week starts on Monday and ends on Sunday, and week 1 is the week containing the year's first Thursday.
How is ISO week 1 defined?
Week 1 is the week that holds the first Thursday of the year. An equivalent rule: it is the week containing January 4. This means week 1 can start as early as December 29 of the previous year or as late as January 4.
What is the difference between ISO and US week numbers?
ISO 8601 weeks start on Monday and use the first-Thursday rule for week 1. The US system starts weeks on Sunday and counts the week containing January 1 as week 1. The two can differ by one near the start and end of a year.
Can a year have 53 ISO weeks?
Yes. A year has 53 ISO weeks when January 1 falls on a Thursday, or on a Wednesday in a leap year. Most years have 52 weeks. The extra week keeps the calendar aligned with the Monday-to-Sunday cycle.
Why do the first days of January sometimes belong to the previous year?
Because week 1 must contain the first Thursday. If January 1, 2, or 3 falls before that Thursday, those days sit in the last week (52 or 53) of the previous ISO year. ISO 8601 treats the week, not the calendar month, as the unit.
Which countries use ISO week numbers?
Most of Europe uses ISO week numbering in business, government and industry, and it is the international standard. The United States and Canada more often use Sunday-start week numbering in everyday contexts, though ISO is common in software and global business.