ISO week number
ISO 8601 weeks start on Monday. Week 1 is the week that contains the year’s first Thursday. The week-year is not always the calendar year.
How to use the ISO week number
What does the ISO week number do?
Convert a date to its ISO 8601 week number and week-year; It applies this page-specific rule: 1 January 2026 is a Thursday, so it sits in week 1 of 2026.
Which inputs does this tool use?
To convert a date to its iso 8601 week number and week-year, enter Date; keep every value in the same example; the page then updates the result from the current values; Start with a familiar case covered by this rule - This is the system used on a lot of European planning boards and in ISO-week date stamps (2026-W35-2) - so the expected result can confirm the settings.
How should the tool result be read?
For a result intended to convert a date to its iso 8601 week number and week-year, this is the system used on a lot of European planning boards and in ISO-week date stamps (2026-W35-2); read date together with the original date rather than as an isolated answer, and confirm that its units or structure match the intended use.
Why is 1 January sometimes week 53?
When the goal is to convert a date to its iso 8601 week number and week-year, if it falls Friday–Sunday, ISO puts it in the previous week-year.
Sunday start?
When using the page to convert a date to its iso 8601 week number and week-year, not this page.
Limits and accuracy
When using this tool to convert a date to its iso 8601 week number and week-year, the result covers the stated calculation and supplied values, not every real-world factor; check the displayed date, time zone, and daylight-saving status before relying on the result for an important appointment; keep date until the output has been checked in its intended destination.
How this works
1 January 2026 is a Thursday, so it sits in week 1 of 2026. Dates at the end of December can belong to week 1 of the next year; dates in early January can belong to week 52 or 53 of the previous year.
This is the system used on a lot of European planning boards and in ISO-week date stamps (2026-W35-2). It is not the US “week 1 = week containing 1 January, Sunday start” convention.
Pick a date. The page prints YYYY-Www and the weekday. Nothing is a timesheet or payroll product.
Frequently asked questions
Why is 1 January sometimes week 53?
If it falls Friday–Sunday, ISO puts it in the previous week-year.
Sunday start?
Not this page. ISO weeks start Monday.
Week 53?
Some years have 53 ISO weeks. That is normal.