Calendar weeks
What week is it? ISO week numbers explained
A week number is a simple way to say which week of the year a date falls in. This guide explains how ISO-8601 week numbers work, why they matter for planning, and how to answer the question 'what week is it?' for any date.
What is an ISO week number?
The ISO-8601 standard numbers the weeks of a year from 1 up to 52 or 53. Every ISO week runs from Monday to Sunday, so a week never splits across two numbers. Week 1 is defined as the week that contains the first Thursday of January, which is the same as saying it is the week with at least four days in the new year.
Because of that rule, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For example, Monday, December 29, 2025 is already the first day of week 1 of 2026.
Why do some years have 53 weeks?
A year has 365 or 366 days, which is 52 weeks plus one or two extra days. Those leftover days add up, and roughly every five or six years an ISO year ends up with a full 53rd week. A year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.
2026 is a 53-week year, while 2027 and 2028 both have 52 weeks. This is why a project plan that counts "week 53" only works in certain years.
ISO weeks compared with US week numbering
In the United States and some other countries, weeks traditionally start on Sunday and week 1 is the week that contains January 1. That means the same date can have a different week number depending on the system. ISO numbering is the standard in Europe, in most international business and in software, so it is the one used on this site.
How to work out what week it is
The quickest way is to look at the live number on the home page, which always shows the current ISO week with its Monday-to-Sunday dates. To check any other day, use the date finder, or open the full calendar for a year and read the red week column next to the month grid.
Doing it by hand: find the Thursday of the week your date falls in, count how many days that Thursday is after January 1, divide by seven, and round up. The result is the ISO week number.
Where week numbers are used
Week numbers are common in project planning, agile sprints, payroll and shift schedules, school and university timetables, shipping and delivery dates, and manufacturing. Saying "delivery in week 34" is shorter and less ambiguous than naming two dates.