Week Number Calculator
Find the ISO week number of any date, and see exactly which days each week covers.
Results
Examples
- 2026-01-01 falls in ISO week 1 of 2026 (2026 starts on a Thursday — a 53-week year).
- 2027-01-01 falls in ISO week 53 of 2026, because it belongs to the week of Monday 2026-12-28.
- 2016-01-03 was still ISO week 53 of 2015.
Frequently asked questions
How is the ISO week number defined?
ISO 8601 weeks start on Monday, and week 1 is the week that contains the year’s first Thursday (equivalently, the week containing January 4). This means January 1–3 can belong to the last week of the previous year, and December 29–31 can belong to week 1 of the next year.
Why do some years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. It happens roughly every 5–6 years; most years have 52.
Is the US week number different?
Often, yes. Many US calendars start weeks on Sunday and call the week containing January 1 “week 1”. Business communication across countries should always specify ISO week numbers to avoid off-by-one confusion.