DATE TOOLS
Week Number
Find the ISO week number for any date — or the dates of any week.
Pick a date above.
How to use the Week Number
- Choose “Date → week” to find which week a date belongs to, or “Week → dates” to find when a given week starts and ends.
- Weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday (ISO-8601).
Frequently asked questions
Why does 1 January sometimes belong to week 52 or 53?
Under ISO-8601, week 1 is the week containing the first Thursday. If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year.
How many weeks are in a year?
Most years have 52 ISO weeks. Years starting on a Thursday — and leap years starting on a Wednesday — have 53.