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ISO week number

Pick a date to see its ISO-8601 week number and week-year.

How ISO week number works

ISO week number gives you the ISO-8601 week (1–53) for any date, together with the ISO week-year, the numbering used across business, manufacturing and European calendars where weeks start on Monday and week 1 is the week containing the first Thursday of the year.

Unlike a general date overview, this tool focuses on getting the ISO week right, including the edge cases at the turn of the year where the week-year differs from the calendar year. For the weekday, quarter or a full breakdown, see the day-of-week and Date details tools. The ISO week-year is shown alongside the number, so the turn-of-year edge cases stay unambiguous.

Frequently asked questions

Why does early January sometimes show week 52 or 53 of the previous year?

Because ISO-8601 ties week 1 to the year's first Thursday. A date in early January can therefore belong to the last ISO week of the previous week-year, which is exactly why the tool shows the ISO week-year alongside the number.

Is this the same week number my spreadsheet shows?

It matches the ISO week (Monday-start, first-Thursday rule). Some spreadsheets default to a US scheme (Sunday-start, Jan 1 in week 1); switch them to the ISO option to get the same result.

Is the date I enter sent anywhere?

No. Every value is computed in your browser, on your device. The date you enter never leaves your machine and nothing is stored between visits.

Does it work offline?

Yes, once the page has loaded. Because the processing happens on your device, you can disconnect from the network and the tool keeps working. This is the clearest proof that your data never leaves your machine.

Is it free? Do I need an account?

It is completely free and requires no sign-up. There are no watermarks, no daily limits and no tracking cookies needed to use the tool.

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