Week Number Calculator
Find the ISO 8601 week number for any date, instantly in your browser.
How to use the Week Number Calculator
- Pick a date from the calendar.
- Click Run to see its ISO week number and week-based year.
- Use the Monday-to-Sunday date range shown to plan or label that week.
How the Week Number Calculator works
ISO 8601 weeks start on Monday. Week 1 of a year is the week that contains the year's first Thursday, which is equivalent to the week containing 4 January. Every week therefore belongs to exactly one year, and most years have 52 weeks while some have 53.
Near New Year a date can belong to the previous or next year's week numbering. For example, 1 January 2027 is a Friday, so it falls in week 53 of 2026.
Frequently asked questions
- Is this tool free?
- Yes. Calculate week numbers at no cost, with no account required.
- What is an ISO 8601 week?
- ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday - so week numbers can belong to a slightly different year than the calendar date near January 1st.
- How many weeks are in a year?
- Most years have 52 weeks, but years where January 1st falls on a Thursday (or a leap year starting on Wednesday) have 53 ISO weeks.