ISO week calculator
Convert a calendar date to an ISO week date, or find the date for a numbered week.
How it works & useful details
How it works & limits
ISO weeks start on Monday. Week 1 contains January 4, so dates near New Year can belong to the previous or following week-year. For example, January 1, 2021 is 2020-W53-5.
Select any day in the week strip to explore its calendar date. Only valid weeks are accepted: not every year has week 53. The complete week must fit inside years 0001–9999.
Your working space
Explore the live preview, then save a copyable text report. Inputs stay on this device and are cleared by refreshing. Dates use the Gregorian calendar, including before its historical introduction.
About this tool
ISO week calculator converts a calendar date into its ISO 8601 week date and does the reverse, finding the dates that belong to a numbered week. It is the tool for sprint and reporting cycles where work is tracked by week rather than by month.
ISO weeks start on Monday and week 1 is the week containing the first Thursday of the year, which is why a January date can belong to week 52 or 53 of the previous year. The conversion is local.
How to use it
- Choose the direction: calendar date to ISO week, or ISO week to date range.
- Enter the date or the year and week number.
- Read the ISO week date, including the week-year when it differs from the calendar year.
- Check the Monday-to-Sunday range for that week.
Worked examples
- 2021-01-01 is 2020-W53-5: the Friday of ISO week 53 of 2020. That week runs from Monday 2020-12-28 to Sunday 2021-01-03.
- 2026-12-31 is 2026-W53-4: 2026 is one of the years with 53 ISO weeks, and its last week ends on 2027-01-03.
The format is year-W-week-day, where day 1 is Monday and day 7 is Sunday.
How ISO weeks are defined
ISO 8601 weeks start on Monday, and week 1 is the week that contains January 4. Equivalently, it is the week with the year's first Thursday. As a result, the first few days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next. The ISO week-year is therefore not always the same as the calendar year, which is a frequent bug in reports that group by week.
Most years have 52 weeks. A year has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Calendars in the United States often number weeks differently, starting on Sunday with week 1 containing January 1, so check which system a schedule uses.
Frequently asked questions
Why is the ISO week-year sometimes different from the calendar year?
ISO weeks are defined by Thursdays, so the first days of January can belong to the last weeks of the previous year and vice versa.
Does every year have 53 weeks?
No. Only years whose first Thursday falls late enough have 53 ISO weeks; the calculator reports 52 or 53 accordingly.
Which weekday starts an ISO week?
Monday. ISO weeks always run from Monday to Sunday.
Why is week 53 rejected for some years?
Only years with 53 ISO weeks have a week 53. For other years the calculator rejects it rather than rolling over into the next year.