“Delivery in week 46.” “Exams start in week 10.” Week numbers are common in factories, shipping, schools and software teams, but many people do not know how they are counted. This guide explains ISO week numbers, why some years have 53 weeks and how to find the current week with our free What Week Is It? tool.
The ISO 8601 rules
- Weeks start on Monday.
- Week 1 is the week that contains the year’s first Thursday (equivalently, the week containing 4 January).
- A year has 52 or 53 weeks.
Why the first Thursday?
Thursday is the middle of a Monday-to-Sunday week. If a week has its Thursday in the new year, most of its days are in the new year, so it counts as week 1.
Consequences
- 1 January can fall in week 52 or 53 of the previous year.
- 29 to 31 December can fall in week 1 of the next year.
- The “ISO year” can differ from the calendar year near the start and end of the year.
Which years have 53 weeks?
A year has 53 ISO weeks if it starts on a Thursday, or if it is a leap year starting on a Wednesday. 2026 starts on a Thursday, so it has 53 ISO weeks.
US and other systems
In the US, many calendars start week 1 on 1 January and begin weeks on Sunday. That can give different week numbers from ISO. If you work with international partners, confirm which system they use.
Where week numbers are used
| Field | Use |
|---|---|
| Manufacturing | Production planning and batch codes |
| Shipping | Delivery schedules |
| Retail | Weekly sales reports |
| Software | Sprint planning |
| Education | Term and exam weeks |
Day of the year
Alongside week numbers, the tool shows the day of the year (1 to 365 or 366). 10 October is day 283 in a common year.
Finding week numbers for other dates
Use the tool’s lookup, or see all weeks at once in the Year Calendar. For time left in the year, see Weeks Left in the Year, and read planning the year by week numbers.
Frequently asked questions
What week number is it today?
The tool shows the current ISO week number.
When does ISO week 1 start?
In the week containing the first Thursday of the year.
Can a year have 53 weeks?
Yes, when it starts on a Thursday, or a Wednesday in leap years.