Week Number Calculator

What Is a Week Number Calculator?

A Week Number Calculator determines which numbered week of the year contains a particular date. This is useful when working with schedules, project plans, business reporting, appointments, school terms, production timetables and other activities that use week numbers rather than individual dates.

This YesTools calculator uses the ISO 8601 week-date system, an internationally recognised method of numbering calendar weeks. Simply select a date and the calculator shows its ISO week number, ISO week year, the Monday on which the week begins and the Sunday on which it ends.

Why Use the Week Number Calculator?

Whether you need to check the current week, verify a project schedule or find the week number for a historical or future date, this calculator provides the relevant calendar information in seconds.

How to Use the Week Number Calculator

You can determine the ISO week number for any date in just a few steps.

1. Select a date — Choose the date you want to check using the date field.
2. Calculate the week — Select “Calculate Week” to determine the ISO week number.
3. Review the result — The calculator displays the ISO week number, ISO week year, selected weekday and the complete Monday-to-Sunday week range.
4. Copy the result if needed — Select “Copy Result” to copy the calculated week information to your clipboard.

Calculate the Week Number for Any Date

Week numbers provide a convenient way to organise dates into recurring seven-day periods. Instead of referring to a specific calendar date, a schedule can refer to Week 12, Week 30 or another numbered week. The Week Number Calculator makes it easy to determine which ISO week contains any selected date.

The calculator follows ISO 8601, where weeks begin on Monday and the first week of an ISO week year is the week containing the first Thursday. This means that the ISO week year does not always correspond exactly to the calendar year shown by the selected date.

What Is ISO Week 1?

ISO Week 1 is the first ISO week of an ISO week year. It is defined as the week containing the first Thursday of the calendar year. Because ISO weeks begin on Monday, Week 1 can begin in December of the preceding calendar year. Similarly, the final ISO week of a year can continue into January of the following calendar year.

Why Can the Week Year Differ From the Calendar Year?

The ISO week-date system groups complete Monday-to-Sunday weeks together. As a result, the first few days of January can sometimes belong to the final ISO week of the previous year, while the final days of December can sometimes belong to Week 1 of the following ISO week year. The calculator reports the ISO week year separately so this distinction is clear.

Understanding ISO Week Numbers

ISO 8601 week numbers range from Week 1 through Week 52 in most years, although some ISO week years contain 53 weeks. Whether a particular year has 52 or 53 ISO weeks depends on how the calendar dates and weekdays align.

This calculator uses calendar dates rather than times of day, so time zones and daylight-saving changes do not affect the result. The calculation is performed locally in your browser, and the selected date does not need to be uploaded to a server.

Frequently Asked Questions