How to use the ISO week number finder
- Choose a Date. When the page opens it holds today’s date, read from the UTC calendar, so near your local midnight it can show yesterday or tomorrow.
- The large result shows the ISO week, for example “Week 40”.
- Below it are the ISO week-year, the ISO week date (year, week and weekday in one code), the weekday and the date range of that week.
- The last lines show how many weeks the ISO year has, the day of the year, and the days left in the calendar year.
What it does and when to use it
Week numbers are common in European workplaces, logistics, school timetables and sprint planning. A delivery note might say “week 11”, or a rota might be built around week numbers. This tool turns any date into that number and shows exactly which Monday and Sunday it covers, so you can match a week number back to real dates.
To find the weekday of a single date, try the day of the week finder. To jump a set number of weeks ahead, use add or subtract days with the unit set to Weeks; it also shows the ISO week of the result. The weekday counter tells you how many Mondays or Fridays fall in a period.
How it works
ISO 8601 defines weeks like this: a week starts on Monday, and week 1 is the week that contains the year’s first Thursday. To place a date, the tool finds the Thursday of the same Monday-to-Sunday week. The year of that Thursday is the week-year. The week number is then the count of days from 1 January of that year to the Thursday, divided by 7 and rounded down, plus 1.
Dates are handled as plain calendar days with no clock time, so your time zone cannot change the week. Only the starting “today” value depends on the clock.
Worked examples
31 December 2026. A Thursday in week 53: ISO week date: 2026-W53-4. The “4” means the fourth day of the ISO week. 2026 starts on a Thursday, so it has 53 weeks.
3 January 2021. A Sunday that belongs to the last week of the previous year: ISO week-year: 2020, week 53, running from 28 December 2020 to 3 January 2021.
15 March 2027. A Monday, so it opens week 11: Week runs: 15 March 2027 to 21 March 2027. 2027 has 52 ISO weeks.
Limits and tips
- The tool uses ISO weeks only. It does not offer US-style weeks that start on Sunday.
- When you write a week date, keep the week-year: “2020-W53” is unclear if written as 2021.
- Valid dates run from year 1 to 9999. Early dates follow the modern Gregorian calendar extended backwards.
Frequently asked questions
Why does 1 January sometimes belong to week 52 or 53?
Which years have 53 ISO weeks?
Is this the same week number Outlook or my phone shows?
What is the week-year?
How do we know nothing is uploaded? Test it yourself on the privacy proof page.