What Week Is It? Week Number Calculator

Wondering what week it is? This page shows the current week number according to the ISO 8601 standard, and lets you look up the week number for any other date. Along with the week, you see the week-based year, the Monday-to-Sunday date range of that week, whether the year has 52 or 53 weeks, and the day of the year.

How to look up a week number

1. Open the page to see today’s week number right away.
2. To check another day, pick a date in the date field.
3. Read the week number and week-based year, for example W14.
4. Check the Monday and Sunday dates of that week and the day of the year.

How ISO week numbers work

ISO 8601 is the international standard used across Europe and in much of international business. Its rules are simple once you know them:

• Weeks start on Monday and end on Sunday.
• Week 1 is the week that contains the first Thursday of the year. Equivalently, it is the week that contains January 4.
• A year has 52 or 53 weeks. It has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.
• 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.

That last point is why the tool shows a week-based year next to the week number. December 31 might be in week 1 of the following year, and January 1 might be in week 52 or 53 of the year before.

US week numbering is different

In the United States, calendars usually start the week on Sunday, and many US systems count week 1 as the week that contains January 1. Under that convention, week numbers can be off by one from the ISO number, particularly near the start or end of the year. Spreadsheet programs offer both systems: in Excel, for example, WEEKNUM uses a Sunday start by default, while ISOWEEKNUM follows the ISO rules. If you exchange week numbers with partners in Europe or Asia, they almost always mean ISO weeks, so it is worth confirming which system everyone uses.

Where week numbers are useful

• Business and finance: weekly reports, sales cycles and fiscal planning.
• Production and logistics: manufacturing schedules and delivery windows are often agreed by week number.
• Software teams: sprint planning and release calendars.
• Schools and universities: semester plans and study schedules organized by week.
• Personal planning: training plans, meal plans or tracking a pregnancy week by week.

A shared week number avoids long date ranges in emails: “delivery in week 23” is short and unambiguous, as long as everyone uses the same standard.

Day of the year

The tool also shows the ordinal day of the year, from 1 on January 1 up to 365, or 366 in a leap year. This number is used in some date formats, in scientific data and in production codes printed on packaging. It is also a handy way to see how much of the year has passed: day 182 or 183 marks roughly the middle of the year.