Find the ISO 8601 calendar week number for any date locally - with the ISO year, weekday and day of the year, no upload.
No. Everything runs in your browser - your file never leaves your device. How this is verifiable
The calendar week number sequentially numbers the weeks of a year. The international standard is ISO 8601: a week starts on Monday, and week 1 is the week that contains the first Thursday of the year (equivalently: the week with 4 January). As a result the first days of January can still belong to the last week of the previous year, and the last days of December can already belong to week 1 of the next year - which is why, alongside the week number, there is a separate ISO week-numbering year. For a given date this calculator shows the calendar week, the ISO year, the weekday and the day of the year.
The calculation runs entirely locally in your browser, in pure JavaScript - nothing is uploaded and nothing is stored. The date is an input (the calculator never reads today's date), the maths is UTC-anchored and therefore deterministic. To determine the week, the Thursday of that week is used, because under ISO 8601 it fixes the week-numbering year. The weekday is shown to match your language. Change the date and everything updates instantly.
An honest note: the calculator follows ISO 8601, the standard common in Europe and much of the world. Some countries, such as the US, count weeks differently (week 1 starts on 1 January, the week begins on Sunday) - there the week number can differ. For business calendars, project planning and appointments in Europe, the ISO calendar week is the right reference.
| Input formats | Form inputs (no file) |
|---|---|
| Processing | Locally in your browser (JavaScript) |
| File upload | None |
Limitations: Week counting per ISO 8601 (week starts Monday, week 1 contains the first Thursday or 4 January). Countries with a different scheme (such as the US, week from Sunday) can yield different numbers. The date is an input, UTC-anchored; no reference to today.
No. The calculation runs entirely locally in the browser (pure JavaScript); nothing is sent or stored.
ISO 8601: the week starts on Monday, and week 1 is the week with the first Thursday of the year (the week containing 4 January).
Because the first days of January can still belong to the last week of the previous year, and the last days of December to week 1 of the next year.
Not necessarily. In the US the week often begins on Sunday and week 1 on 1 January; there the week number can differ.