How to use it
- Read the current week number at the top.
- Pick any date to see its week number.
- Or enter a year and week number to see that week's dates.
How ISO week numbers work
The ISO 8601 standard, used in business, manufacturing, logistics and most of Europe, numbers weeks like this:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4).
- A year has 52 or 53 weeks.
Because of these rules, the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next year. The calculator points this out when it happens.
ISO weeks vs. U.S. calendar weeks
Many U.S. calendars and planners start the week on Sunday and count the week containing January 1 as week 1. Under that system, week numbers can differ from ISO by one, especially early and late in the year. Spreadsheet apps support both: in Excel, =ISOWEEKNUM(date) gives the ISO week, while =WEEKNUM(date) defaults to the Sunday-start U.S. system.
If you work with international colleagues, suppliers or software (for example, sprint planning or shipping schedules), ISO week numbers are usually what they mean by "week 23".
Look up any week
- Date → week: pick a date to see the ISO year and week number, plus the Monday-to-Sunday dates of that week.
- Week → dates: enter a year and a week number to see when that week starts and ends. For example, ISO week 1 of 2027 runs from Monday, January 4 to Sunday, January 10, 2027.
The top of the page also shows how many ISO weeks the current year has.
Which years have 53 weeks?
A year has 53 ISO weeks if it starts on a Thursday, or if it's a leap year that starts on a Wednesday. That happens about 71 times in 400 years, roughly every five to six years. Recent and upcoming 53-week years include 2020 and 2026; the next one after that is 2032.
Where week numbers are used
| Field | Example |
|---|---|
| Project management | "The release is planned for week 42." |
| Manufacturing and retail | production calendars, sales reports |
| Shipping and logistics | delivery windows by calendar week |
| Schools and payroll | biweekly schedules, odd and even weeks |
The calculator uses your device's date and runs entirely in your browser.
Frequently asked questions
What week number is it today?
The large number at the top of the page is today's ISO week number, with the Monday-to-Sunday dates of the current week.
When does week 1 start?
Under ISO 8601, week 1 is the week with the year's first Thursday, so it starts on the Monday between December 29 and January 4.
Why is January 1 in week 52 or 53?
If January 1 falls on a Friday, Saturday or Sunday, that week belongs to the previous ISO year.
Do U.S. week numbers differ from ISO?
They can. U.S. calendars often start weeks on Sunday and count the week of January 1 as week 1, which can shift numbers by one.
How many weeks are in a year?
52 or 53 ISO weeks. 2026 has 53.
Last updated: November 12, 2026