Week Number Calculator

Find the ISO week number for any date. See which week of the year a specific date falls in.

1. Enter a date using the date picker to find which ISO 8601 week number it falls in. 2. View the week number along with the Monday start date and Sunday end date of that ISO week. 3. Check how many total ISO weeks exist in the selected year - either 52 or 53 depending on the year. 4. Browse the full yearly week calendar below to see every week number and its date range at a glance. 5. Optionally enter a specific year and week number to look up the corresponding Monday-to-Sunday date range.

About This Tool

The Week Number Calculator determines the ISO 8601 week number for any given date. Simply enter a date and the tool instantly tells you which week of the year it belongs to, along with the start and end dates of that week.

ISO week numbering is used internationally in business, project management, manufacturing, and logistics. The ISO standard defines week 1 as the week containing the first Thursday of the year, and weeks always start on Monday.

The calculator also shows the total number of ISO weeks in the selected year (52 or 53) and lets you browse weeks for an entire year at a glance. This is helpful for planning schedules, setting deadlines, and aligning calendars across teams.

Frequently Asked Questions

ISO 8601 defines a standard week numbering system. Weeks start on Monday and the first week of the year is the one containing the first Thursday of January. This means week 1 always contains January 4th.

Yes. Most years have 52 ISO weeks, but years that start on a Thursday, or leap years that start on a Wednesday, have 53 weeks. This happens roughly every 5 to 6 years.

Under ISO 8601, if January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year. The first ISO week always contains at least 4 days in the new year.

ISO 8601 week numbering is an international standard, but some countries (notably the US) use a different system where weeks start on Sunday and week 1 is the week containing January 1.

Enter the year and week number into the calculator and it will show you the Monday-to-Sunday date range for that specific ISO week.

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