Week Number Calculator
Find the ISO week number for any date and the dates that week covers.
About this week number calculator
Many businesses, schools and manufacturers plan in numbered weeks. The international standard, ISO 8601, defines weeks as starting on Monday, with week 1 being the week that contains the year's first Thursday. This calculator gives the ISO week number for any date and the Monday-to-Sunday dates of that week.
Because of the Thursday rule, the first few days of January can belong to the last week of the previous year, and late December can belong to week 1 of the next year — the calculator shows the correct ISO week-year.
How to use the Week Number Calculator
- Choose a date (defaults to today).
- Read the ISO week number, the week's dates and the number of weeks in that year.
How ISO week numbers work
Weeks start on Monday. Week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4). Most years have 52 weeks; years that start on a Thursday, or leap years that start on a Wednesday, have 53.
Worked example
October 14, 2026 falls in ISO week 42, which runs from Monday, October 12 to Sunday, October 18, 2026.
Tips for better results
- Some US calendars number weeks starting on Sunday with week 1 containing January 1, so numbers can differ by one from ISO weeks.
Frequently asked questions
What week number is it today?
Open the calculator — it shows today's ISO week number by default.
Why does January 1 show week 52 or 53?
Under ISO 8601, if January 1 falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year.
Last reviewed: October 2026 · Results are calculated in your browser.