Week Number Calculator
Find the ISO 8601 week number (1 to 53) for any date you choose.
Date
2026-W25 — Thursday
25
2026
2026-W25
Thursday
169
Formula
Examples
| Input | Result |
|---|---|
| 2026-01-01 (Thursday) | Week 1 of ISO year 2026 |
| 2027-01-01 (Friday) | Week 53 of ISO year 2026 |
About this calculator
The week number calculator returns the ISO 8601 week number for any date. Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday — equivalently, the week containing January 4. Years have either 52 or 53 ISO weeks.
Because the first and last days of a calendar year can belong to a week in the neighbouring year, the calculator also reports the "ISO week year", which may differ from the calendar year. For example, the last days of December can fall in week 1 of the next ISO year, and early January can fall in week 52 or 53 of the previous one.
Frequently asked questions
Weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of the year (equivalently, the week containing January 4).
Days at the very start or end of a calendar year can belong to a week assigned to the adjacent year, so late December may be week 1 of next year and early January may be week 52 or 53 of last year.
Yes. An ISO year has 53 weeks when it starts on a Thursday, or starts on a Wednesday in a leap year; otherwise it has 52.
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