Find the ISO 8601 week number for any date, along with the day of the year, the standard used across business and European calendars.
How it works
- Select a specific calendar date.
- The engine calculates the week based on ISO 8601 Thursday rules.
- Read the week number and the day-of-the-year figure.
Frequently asked questions
What is an ISO week?
A Monday-to-Sunday week numbered so that week 1 contains the first Thursday of the year.
Why can early January be week 52 or 53?
If the first days of January fall before that year's first Thursday, they belong to the final week of the previous year.
Who uses week numbers?
Business planning, manufacturing, payroll, and much of Europe schedule by ISO week number.
Why does week 1 sometimes start in the previous year?
Under ISO 8601, week 1 is the week containing the year's first Thursday. If January 1 falls late in the week, it belongs to week 52 or 53 of the previous year.
Does the US use the same week numbering as Europe?
No — the US traditionally starts weeks on Sunday and counts from January 1, while ISO 8601 (used in Europe) starts weeks on Monday and defines week 1 as the week containing the first Thursday.