ISO Week Date Calculator
Find the ISO 8601 week number, week-numbering year and weekday for any date — plus the full YYYY-Www-D week date used in business calendars.
How it works
ISO weeks start on Monday · week 1 is the week containing the year’s first Thursday · the week-numbering year is the year of that week’s Thursday · weekday runs 1 = Monday to 7 = Sunday · format is YYYY-Www-D
The ISO 8601 week date system numbers weeks so that each week belongs entirely to one year. Weeks begin on Monday, and week 1 is the one holding the year’s first Thursday — equivalently, the week containing January 4. That is why the last days of December can fall in week 1 of the next year, and early January can fall in week 52 or 53 of the previous year, giving a "week-numbering year" that may differ from the calendar year. Most years have 52 weeks; some have 53.
Worked example
September 15, 2026 is a Tuesday, so its ISO weekday is 2. That week’s Thursday is September 17, 2026, placing it in week 38 of 2026 — the full ISO week date is 2026-W38-2.
Sources: ISO week date — Wikipedia
Frequently asked questions
Why can a date in late December be week 1 of next year?
Because ISO week 1 is defined by the first Thursday. If December 29–31 fall in the same Monday-start week as the next year’s first Thursday, they are counted as week 1 of that next year.
What is the difference between ISO weeks and ordinary week numbers?
Many calendars start weeks on Sunday and call the week containing January 1 "week 1". ISO weeks start on Monday and use the first-Thursday rule, so the numbers can differ by one, especially at the start and end of the year.
When does a year have 53 ISO weeks?
When January 1 is a Thursday, or when it is a Wednesday in a leap year. Those years pack an extra full week, giving 53 instead of the usual 52.
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