Week number

The ISO 8601 week of a date, the days of a week, and all the weeks of a year.

This week
Week of a date

Days of a week

You can also paste a notation such as 2026-W40 into the week field.

    All the weeks of
    WeekMondaySundayFrom – to

    Which week number?

    ISO 8601: Europe, most calendars and software
    A week runs from Monday to Sunday. Week 1 is the one that contains the first Thursday of the year (equivalently, 4 January). A year has 52 or 53 weeks, and the first days of January can belong to the last week of the previous year: 1 January 2021 is in week 53 of 2020. This is what date +%V and %G give.
    US convention
    A week runs from Sunday to Saturday, and week 1 is the one that contains 1 January, even if it only has one day of the new year. The numbers differ from ISO most of the year, and go up to 54. Always say which convention a week number follows.

    Transparency

    Everything is computed in your browser, by the scripts listed below. Nothing you type leaves this page.

    Enforced by your browser
    This page's Content Security Policy allows a single destination, the site's audience measurement (connect-src https://stats.r-no.fr, see below), and forbids any form submission (form-action 'none'), so even a bug could not send what you type anywhere. You can read the policy at the top of the page source.
    Audience measurement
    Like every page of the site, this one counts its visit with Umami, installed on R-no’s own server (stats.r-no.fr). It receives the address of the page, without parameters or anchor, the site you came from (its domain only), your browser, system, language and screen size, and the country, region and city deduced from your IP address, which is not kept. It never receives what you type, nor the result of the tool. No cookie, nothing stored in your browser; if your browser asks not to be tracked (Do Not Track), nothing is measured. The data is erased after 25 months.
    • /commun/mesure.js: the filter applied before each sending, written for R-no Toolbox. 1.4 kB
      SHA-256 225eec74fd12623d82fc05ee7d222be629ce454d081c129e8d1e249ed37ee716
    • https://stats.r-no.fr/m.js: Umami’s tracker, open source (MIT), served by the measurement server. Its fingerprint is not shown here: it changes with each version of Umami, and you cannot check from your browser that it is the published code. For that part, you have to trust R-no.
    What is stored
    No cookie, and nothing in the address. What you type is never stored: it disappears when you reload or close the page. Your language is given by the address itself (/en/ or /fr/): nothing is stored to remember it. Like any website, the server records the download of the page and its files in its access logs (IP address, date and time, address requested, browser name). It never receives what you type.
    Code that runs here
    • app.js: the week calculations and the page, written for R-no Toolbox. Not minified, no third-party library. 13.4 kB
      SHA-256 f6d49254cd22981f65f1f879a9163f6b6300cac76aec6dc294d46547e88c1b07
    • textes.js: the French messages of the tool, shown by its script (the rest of the French page is written into its HTML when the site is built). It only defines texts, with no logic. 6.0 kB
      SHA-256 8ce57726eefa425f15f2cf8c292dd6eed8df9ddc47cf9a4e551cf628d401050e
    • /commun/langue.js: the language helper, the same on every page of the site. It reads the language of the page and gives the tool its messages in that language. It stores nothing. 1.6 kB
      SHA-256 80f76f69773ef628d36d09bf8387cfedebda4529210d7120649b6f5603310631
    How it is computed
    Each date is turned into a day number (1 January 1970 = 0) with integer arithmetic only: no time, no time zone, so daylight saving changes cannot shift a day. The week is that of the Thursday of the same Monday-to-Sunday week, as ISO 8601 defines it.
    How it was checked
    The ISO week and year were compared with GNU date (%G-W%V-%u) on every day from 1 January 1900 to 31 December 2100: 73,414 identical results. The tests run again before each release.
    Check it yourself
    Download a script and compare its fingerprint with the one above:
    curl -s https://r-no.fr/dates/week/app.js | shasum -a 256
    Limits
    Gregorian calendar, extended before 1582 as if it had always been in use, from year 1 to 9999. “Today” and “This week” come from your device’s clock and time zone. Other conventions exist (the Middle East, for instance, often starts weeks on Saturday): only ISO 8601 and the US convention are shown.