Week Number Calculator
Pick a date, or leave it on today, and we tell you which week of the year it falls in.
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The formula
ISO week: weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of the year.
ISO weeks, in plain words
Most of Europe, and many companies worldwide, use ISO 8601 weeks. A week starts on Monday, and week 1 is the one containing the first Thursday of January. That means the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next year.
For example, 1 January 2026 is a Thursday, so it is in week 1 of 2026, while 31 December 2026 falls in week 53.
Why week numbers are used
Production schedules, shipping, school terms, payroll runs and sprint planning are often labelled by week number because it is short and unambiguous.
The US system is different
In the United States, weeks often start on Sunday and week 1 contains 1 January. That can give a different number from the ISO week, especially near the start and end of the year. This tool uses ISO weeks.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Date: 2026-01-01 | ISO week number: Week 1 of 2026 |
| Date: 2026-06-15 | ISO week number: Week 25 of 2026 |
| Date: 2026-12-31 | ISO week number: Week 53 of 2026 |
| Date: 2027-01-03 | ISO week number: Week 53 of 2026 |
Week Number Calculator: frequently asked questions
How many weeks are in a year?
An ISO year has 52 or 53 weeks. A year has 53 when it starts on a Thursday, or on a Wednesday in a leap year.
What week is it right now?
Leave the date on today and the answer is shown straight away.
Does it take leap years into account?
Yes. Calculations use real calendar dates, including leap days and the different lengths of months.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026