Find the ISO 8601 week number for any date, plus the weekday and ISO year.
Calculated locally in your browser.
How do you find the ISO 8601 week number for a date?
Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday, equivalently the week holding 4 January. Because weeks belong to whichever year holds their Thursday, the ISO year can differ from the calendar year near New Year. For example, 1 January 2021 falls in ISO week 53 of 2020.
Understanding your result
Because weeks are assigned to whichever year holds their Thursday, the ISO week-numbering year can differ from the calendar year for a few days around New Year. Most years have 52 weeks; some have 53. This is the week numbering used across Europe and in many business and software systems.
Formula and method
Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday (equivalently, the week containing 4 January).
Worked example
1 January 2021 is a Friday and falls in ISO week 53 of 2020, because that week's Thursday is still in 2020.
How to use this tool
- Pick a date, or leave it blank for today.
- Press Calculate.
- Read the ISO week number, weekday and ISO year.
Common mistakes to avoid
- Assuming weeks start on Sunday — ISO weeks start on Monday.
- Expecting the ISO year to always match the calendar year near 1 January.
- Confusing the ISO week number with a simple "day ÷ 7" count.
About the Week Number Calculator
The Week Number Calculator tells you which ISO 8601 week any date falls in, along with the weekday, the ISO week-numbering year and how many weeks that year has.
Who should use this tool
Project managers, schedulers, accountants and developers working with week-based planning.
Benefits
- Accurate ISO 8601 week number for any date.
- Shows the weekday and ISO week-numbering year.
- Tells you whether the year has 52 or 53 weeks.
- Private — calculated entirely in your browser.
Practical use cases
- Referring to a sprint or pay period by week number.
- Planning schedules that run on ISO weeks.
- Checking the week number for reports or invoices.
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Frequently asked questions
What is the current week number?
Leave the date blank and press Calculate to get today's ISO 8601 week number.
Why can the year differ from the calendar year?
A week belongs to the year that contains its Thursday, so the first or last few days of a calendar year can fall in the previous or next ISO week-numbering year.
Can a year have 53 weeks?
Yes. A year has 53 ISO weeks when it starts on a Thursday, or when a leap year starts on a Wednesday.