Week Number Calculator
What week of the year is this date?
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Businesses, schools, and project plans often refer to dates by their week number — 'we ship in week 34'. Pick any date and this calculator returns its ISO week number, plus the day of the year, the quarter, and the weekday.
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What Week Number Is This Date? — The Quick Answer
The ISO standard numbers weeks from 1 to 52 (occasionally 53), where week 1 is the week containing the first Thursday of the year and each week runs Monday to Sunday.
Shortcut
ISO week 1 = the week (Mon–Sun) containing the year's first Thursday
How a year breaks into weeks
| Period | ISO weeks |
|---|---|
| Early January | Week 1–2 |
| Mid-year | Week 26–27 |
| Most of December | Week 49–52 |
| A long year | up to week 53 |
Week numbering isn't universal. ISO 8601 (used in most of Europe and by many software systems) starts weeks on Monday. The United States convention often starts on Sunday and numbers differently, so a US 'week 1' may not match ISO 'week 1'.
How It Works
The Method
ISO 8601 week numbering
Formula
week 1 = the week (Monday–Sunday) that contains the year's first Thursday week number = weeks elapsed since that week, plus 1
Variables
The anchor
ISO defines week 1 as the week containing the first Thursday of January. Using Thursday makes the rule consistent year to year, since it guarantees week 1 always contains at least four days of the new year.
Week boundaries
In ISO, every week runs from Monday to Sunday. This differs from the US Sunday-start convention, which is the main reason week numbers can disagree between systems.
Note: A date in early January can belong to the last week (52 or 53) of the previous year, and a date in late December can fall in week 1 of the next year. The calculator assigns the ISO week year accordingly, which is why the ISO year shown may differ from the calendar year in those edge cases.
Last updated: October 5, 2026
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Frequently Asked Questions
What week number is it today?
Enter today's date and the calculator returns the current ISO week number. ISO weeks run 1 to 52, or 53 in a long year, with week 1 being the week that contains the year's first Thursday.
What is the ISO week number?
ISO 8601 is an international standard for dates that numbers the weeks of the year from 1 to 52 (or 53). Week 1 is the week containing the first Thursday of the year, and every week runs Monday through Sunday — making it consistent across countries and software systems that follow the standard.
Why does my calendar app show a different week number?
Because week-numbering conventions vary. ISO 8601 (most of Europe, many business systems) starts weeks on Monday and uses the first-Thursday rule. The United States often starts weeks on Sunday, and some systems count week 1 as the week containing 1 January. Check which convention your calendar or contract uses.
Can there be a week 53?
Yes. In years when the first Thursday rule pushes an extra week in — which happens roughly every five to six years — the ISO calendar has 53 weeks. The calculator handles these automatically and will show week 53 when it applies.