How to use the Week Number Calculator (ISO Week)
- Pick a date, or leave today’s date in place.
- Read the ISO week number and its ISO year.
- See the weekday and the day number within the year.
- Check the days left in the year for planning.
Formula used
ISO week: take the Thursday of the date’s week (weeks start Monday). Its year is the ISO year, and the week number is the count of weeks from the week containing 4 January.
Worked example
Here is the Week Number Calculator (ISO Week) worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| Result | |
| ISO week number | Week 44 of 2026 |
| Day of the week | Friday |
| Day of the year | Day 303 of 365 |
| Days left in the year | 62 |
The ISO week rules
In ISO 8601, weeks run Monday to Sunday and week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January. For example 30 October 2026 is a Friday, week 44 of 2026, and day 303 of 365.
The system is used across Europe and in many international businesses. It means every week belongs entirely to one ISO year.
Why the ISO year can differ from the calendar year
Dates near New Year can belong to the neighbouring year. 30 December 2024 is in week 1 of 2025, and 3 January 2021 is in week 53 of 2020. In the same way, 1 January 2027 is in week 53 of 2026, while 1 January 2026 is in week 1 of 2026.
This matters when you filter reports by week number: the first and last days of a calendar year may sit in a week labelled with a different year.
52 or 53 weeks
Most years have 52 ISO weeks. A year has 53 when 1 January is a Thursday, or when it is a leap year starting on a Wednesday. 2020 and 2026 are examples with 53 weeks.
The extra week affects payroll, retail and any plan that assumes a fixed 52-week year.
Other systems
The United States and some other countries number weeks differently, often with weeks starting on Sunday and week 1 containing 1 January. Calendar apps and spreadsheets may use either system, so numbers can differ by one.
If you need to match a specific system, check its definition. The day of the year shown here counts 1 January as day 1, including 29 February in leap years.
Frequently asked questions
What week is it now?
Leave the date set to today and the calculator shows the current ISO week.
What is week 1?
In ISO 8601, the week containing the first Thursday of the year, equivalent to the week with 4 January.
Do weeks start on Sunday or Monday?
ISO weeks start on Monday. Some systems, such as in the US, start on Sunday.
Can a year have 53 weeks?
Yes. Years like 2020 and 2026 have 53 ISO weeks.
Why is 1 January sometimes in week 52 or 53?
If it falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous ISO year.
Is the day of the year affected by leap years?
Yes. In a leap year there are 366 days, and days after 28 February are numbered one higher.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.