Week Number
Find the ISO 8601 week number of today or any other date, and browse the complete week calendar of any year. Free and instant — everything is calculated right in your browser.
All week numbers of
| Week | From (Monday) | To (Sunday) |
|---|
The highlighted row is the week of the date selected above.
How to use the Week Number tool
- Check today's week. The tool opens with today's date already selected, so the big number at the top is the current ISO week number.
- Look up any other date. Pick a date to see its week number, its ISO week-year, the Monday–Sunday range of that week, the day of the year and how many weeks the year has.
- Browse the year calendar. The table lists every week of the chosen year with its start and end date. Use the year field or the Previous/Next buttons to jump between years.
About this tool
Week numbers are widely used in business planning, manufacturing schedules, school timetables and project management across Europe and beyond — "delivery in week 34" is often clearer than a date range. This week number calculator follows the international ISO 8601 standard, where weeks start on Monday and week 1 is the week that contains the first Thursday of January (equivalently, the week containing 4 January). That is the same numbering used by spreadsheet functions like ISOWEEKNUM.
Besides the current week number, the tool shows the full week calendar for any year from 1 to 9999, including years with 53 weeks, and tells you exactly which dates each week covers. Everything is computed locally in your browser — no date you enter is ever sent to a server.
Frequently asked questions
How is the ISO week number defined?
Under ISO 8601, weeks run from Monday to Sunday, and week 1 of a year is the week containing the first Thursday of January — which is also the week containing 4 January. That means up to three days at the start of January can still belong to the last week of the previous year, and vice versa.
Why does the ISO week-year differ from the calendar year sometimes?
Dates around New Year can belong to a week that is counted in the neighbouring year. For example, if 1 January falls on a Friday, it belongs to the last week (52 or 53) of the previous ISO year. That is why the tool shows a separate "ISO week-year" value.
When does a year have 53 weeks?
An ISO year has 53 weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year. This happens roughly every 5–6 years — 2015, 2020 and 2026 are examples of 53-week years. All other years have exactly 52 weeks.
Do American calendars use the same week numbers?
Not always. Many North American calendars start weeks on Sunday and call the week containing 1 January "week 1". That system can differ from the ISO number by one week. This tool uses the ISO 8601 standard, which is the international norm and the one used in most business contexts.