About this tool
The ISO week number tells you which numbered week of the year a date falls in, under the ISO 8601 standard. Weeks start on Monday and the week containing the first Thursday of the year is considered week 1 — a convention used widely in Europe and in business reporting.
This tool gives you the current week number plus the day-of-year, the quarter, the percent of the year elapsed, and a small Monday-to-Sunday calendar for the current week.
How to use it
- 1Open the tool — today's week number appears immediately.
- 2Change the date to see the week number for any past or future day.
- 3Review the day-of-year and quarter for additional context.
- 4Glance at the week calendar to see which days fall into the current ISO week.
Common use cases
- Project planning and sprint scheduling by week
- European business reporting (e.g. "this issue ships in W42")
- Logging and analysing data over weekly periods
- Coordinating with international colleagues who use week numbers
- Academic calendars and HR systems that track weekly cycles
How it works
The calculation follows ISO 8601: week 1 is the week containing the first Thursday of the year. This means a year can have either 52 or 53 weeks. A year that starts on Thursday, or a leap year that starts on Wednesday, has 53 weeks.
Frequently asked questions
Why does ISO start the week on Monday?
ISO 8601 was designed in part for European business calendars where Monday is the conventional start of the working week. In the US, week numbering systems often start on Sunday and use slightly different rules.
How many weeks are in a year?
Either 52 or 53. Most years have 52; years like 2020 and 2026 have 53 weeks.
What is the difference between ISO week and "week of year"?
ISO week follows the Monday-start, first-Thursday rule. Some other systems count from January 1, which can produce a "partial" week 1 of just a few days.
Why is January 1 sometimes in week 52 or 53 of the previous year?
Because ISO week 1 is the first week with at least 4 days in the new year. If January 1 is on a Friday, Saturday, or Sunday, it belongs to the last ISO week of the previous year.