What are ISO week numbers?
ISO week numbers are a standardized system defined by ISO 8601 for identifying weeks within a calendar year. Under this system, every week begins on Monday and ends on Sunday, and Week 1 is defined as the week containing the first Thursday of the year. This rule has an important consequence: the first days of January can belong to the last ISO week of the previous year, and the last days of December can belong to ISO Week 1 of the following year. For example, if January 1 falls on a Friday, Saturday, or Sunday, those days are counted as part of the final week of the preceding year. ISO week numbering is the international standard used across Europe, in international manufacturing, logistics, payroll, and project scheduling. Businesses in Germany, France, the Netherlands, and Scandinavia routinely reference weeks by their ISO number in contracts, delivery schedules, and production plans. The system eliminates ambiguity when communicating across month and year boundaries. A year contains either 52 or 53 ISO weeks: most years have 52, but a year gains a 53rd week when January 1 falls on Thursday, or on Wednesday in a leap year. The week-based year (sometimes called the ISO year or ISOWeekYear) can differ by up to three days from the Gregorian calendar year.
How to use week numbers in 2025
Week numbers in 2025 are especially useful for scheduling, planning, and compliance tasks. Project managers use ISO week numbers to assign sprint numbers, milestone dates, and delivery windows without ambiguity — saying "deliver by W14 2025" is clearer than citing a date that may shift based on holidays. In the European Union, employment law in many member states uses ISO week numbers for shift planning, overtime calculation, and statutory rest periods. Fiscal calendars in retail and manufacturing often divide the year into 13 four-week periods aligned to ISO weeks. For 2025, you can use the calendar grid above to instantly look up which week any given date falls in, and the week table below gives you the precise Monday–Sunday range for each of the 52 weeks. If you are coordinating across teams in different countries, ISO week numbers provide a single shared reference that works regardless of local holiday differences. Whether you are planning a product launch, scheduling deliveries, preparing payroll, or running a multi-week marketing campaign, the ISO week calendar for 2025 on this page gives you a complete, printable reference.
Frequently asked questions
How many weeks are in 2025?
2025 has 52 ISO weeks. ISO weeks run Monday to Sunday, and the count of weeks in a year is 52 or 53 depending on what day January 1 falls on. A year has 53 weeks when January 1 is a Thursday, or a Wednesday in a leap year.
What is ISO week 1 of 2025?
ISO Week 1 of 2025 is the week containing the first Thursday of 2025. It starts on 30 December 2024 and ends on 5 January 2025. Week 1 is defined this way by ISO 8601 so that the majority of days in Week 1 always fall within the new year.
What week number is it today?
You can find today's ISO week number by using the week number calculator on this site. If you are viewing the 2025 calendar, the current week row is highlighted automatically in the table below.
How are ISO week numbers calculated?
To calculate the ISO week number for a date: (1) find the Thursday of the same ISO week by adjusting the date if necessary; (2) compute the ordinal day of that Thursday within its calendar year; (3) divide by 7 and round up. The ISO year of a date may differ from its Gregorian year for dates in late December or early January.
What is the difference between ISO weeks and US weeks?
ISO weeks start on Monday and Week 1 contains the first Thursday of the year. US weeks start on Sunday, and Week 1 is simply the week containing January 1. This means the two systems can disagree by one week, especially in late December and early January. Most of Europe and international business uses ISO week numbers.
When does week 1 of 2025 start?
ISO Week 1 of 2025 starts on 30 December 2024. This is the Monday of the week that contains the first Thursday of 2025, as specified by ISO 8601.