Enter a date to see its ISO 8601 week number, week-year, weekday and day of the year, or enter a year and week to find the dates it covers.
Results are estimates for planning. Check the figures before you make a financial or health decision.
How the Week number calculator works
The calculator uses the ISO 8601 system, which the UK and most of Europe follow.
- Weeks run from Monday to Sunday.
- Week 1 is the week that contains the year’s first Thursday, which is the same as the week containing 4 January.
- A week belongs to the year that contains its Thursday. That means early January dates can fall in week 52 or 53 of the previous year, and late December dates can fall in week 1 of the next.
- A year has 52 weeks, or 53 if 1 January is a Thursday, or a Wednesday in a leap year.
For a date, the tool shows the week, week-year, weekday, day of the year, and the Monday and Sunday of that week. For a year and week, it lists every day of the week. All maths uses UTC. Note that other systems differ: in the US, weeks usually start on Sunday and week 1 is the week with 1 January, so numbers can disagree by one.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| What do you want | Week number of a date |
| Date | 2026-10-05 |
Monday, 5 October 2026: Week 41
| Weekday | Monday |
|---|---|
| ISO week-year | 2026 |
| Day of the year | 278 of 365 |
| Week starts (Monday) | Monday, 5 October 2026 |
| Week ends (Sunday) | Sunday, 11 October 2026 |
| Weeks in that ISO year | 53 |
Tips and common mistakes
Check which system your employer, school or software uses before you quote a week number, since US-style numbering can differ by one. Remember that week 1 may start in late December and week 53 may end in early January. For planning, use the week-year rather than the calendar year to label a week. Use the date mode to find which week a deadline falls in, and the reverse mode to see the dates of a week such as week 20.
Frequently asked questions
What week number is it today?
Enter today’s date in the calculator and it shows the ISO 8601 week and week-year. The date is entered by you, so the result is always for the date shown.
Why is 1 January sometimes in week 52 or 53?
Because week 1 is the week with the first Thursday. If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year.
Which years have 53 weeks?
Years where 1 January is a Thursday, or a Wednesday in a leap year, have 53 ISO weeks. The calculator shows the number of weeks in each year.