What is an ISO week number?
ISO 8601 defines a standardized system for numbering weeks within a year. Week 1 is the week containing the first Thursday of the year, and weeks always run Monday through Sunday. This rule means that week 1 of a new year can actually begin in late December of the previous year — and conversely, the final days of December may belong to week 1 of the following year. For example, December 28 is always in the last week of its ISO year, while January 1 might be in week 52 or 53 of the prior year. This system is widely used in European business, logistics, manufacturing, and project management to create unambiguous week references that span year boundaries without confusion.
When does week 1 start?
ISO week 1 starts on the Monday of the week that contains January 4th of the new year. A simpler way to remember it: week 1 is the first week of the year that has at least four days falling in January. Because of this rule, years can have either 52 or 53 ISO weeks. A year has 53 weeks when January 1 falls on a Thursday (or on a Wednesday in a leap year). The week numbering system used in the United States differs from ISO: the US system counts Sunday as the first day of the week and week 1 as the week containing January 1, regardless of which day of the week that falls on.
Frequently asked questions
What week number is it this week?
The calculator shows the current ISO week number automatically when you open the page — no input needed. ISO weeks run Monday to Sunday and there are 52 or 53 per year. You can also enter any date to find its week number.
How many weeks are in a year?
Most years have 52 ISO weeks. Some years have 53 weeks — this happens when January 1 falls on a Thursday, or on a Wednesday in a leap year. The years 2026, 2032, and 2037 are upcoming 53-week years.
What is the difference between ISO week numbering and US week numbering?
ISO 8601 weeks start on Monday, and week 1 contains the year's first Thursday. The US system starts weeks on Sunday, and week 1 is simply the week that contains January 1. This means the two systems can differ by up to a week, especially at the start and end of the year. Most international business and European countries use ISO week numbers.
When does week 1 start in 2026?
ISO week 1 of 2026 starts on Monday, December 29, 2025 and ends on Sunday, January 4, 2026. This is because January 1, 2026 falls on a Thursday, which is within that Monday-to-Sunday span.
Why does week 1 sometimes start in December?
ISO 8601 defines week 1 as containing the first Thursday of the year. If January 1 falls on a Friday, Saturday, or Sunday, those days belong to week 52 or 53 of the previous year, and week 1 of the new year starts on the preceding Monday in December.
Does every year have 52 weeks?
Most years have 52 ISO weeks. Years where January 1 falls on Thursday (or Wednesday in leap years) have 53 weeks. You can verify this by entering December 31 of any year into the calculator to see its week number.
What week number is it in week-based project planning?
In project management and manufacturing, week numbers are used to schedule deliveries, sprints, and milestones without specifying exact dates. ISO week numbers are the international standard for this. Enter any project date above to find its week number instantly.
What is week number 1 of 2025?
ISO week 1 of 2025 starts on Monday, December 30, 2024 and ends on Sunday, January 5, 2025. Enter 01/01/2025 in the calculator above to confirm — it will show week 1 of 2025.