DocomintCalculatrice de Numéro de Semaine
Trouvez le numéro de semaine ISO 8601 d'une date, la norme utilisée en entreprise et en gestion de projet.
What is a Calculatrice de Numéro de Semaine?
Uses the real ISO 8601 week-numbering rule (weeks run Monday–Sunday, and week 1 is whichever week contains the year's first Thursday) rather than a naive day-count-divided-by-seven approximation, which misdates the first or last few days of many years — the ISO week's year can differ from the calendar year right around New Year's, which this handles correctly.
How to use the Calculatrice de Numéro de Semaine
- Enter any date
- Press Find Week Number
- The ISO 8601 week number and its associated ISO year appear instantly, computed on your device
Formula
ISO Week 1 = the week (Monday–Sunday) containing the year's first Thursday
Every other week counts forward or backward from week 1 by that definition — a small number of years end up with 53 weeks rather than 52 as a result.
Example
August 8, 2026 falls in ISO week 32 of 2026 — but January 1, 2027 (a Friday) actually falls in ISO week 53 of 2026, not week 1 of 2027, since it's part of the week containing December 31's Thursday.
Common mistakes
- Assuming ISO week numbers match a naive "day of year ÷ 7" calculation — the ISO rule is anchored to the first Thursday, not January 1st.
- Using this interchangeably with a US-style, Sunday-starting week count from a calendar app — those aren't the same numbering system.
- Not realizing the ISO week's associated year can differ from the calendar year for dates right around New Year's.
Frequently asked questions
Why can the ISO week's year differ from the calendar year?
Because ISO 8601 defines week 1 as whichever week contains the year's first Thursday — a date in late December can fall in "week 1" of the following year, and a date in early January can fall in the final week (52 or 53) of the previous year, depending on which weekday January 1st lands on.
Do ISO weeks start on Sunday or Monday?
Monday — ISO 8601 defines the week as running Monday through Sunday, unlike the US convention of a Sunday-starting week used in many calendar apps.
Why doesn't this match the week number in my calendar app?
Many calendar apps default to a US-style Sunday-starting week count rather than the ISO 8601 standard (Monday-starting, anchored to the year's first Thursday) this tool uses — check whether your app has an ISO week option to compare like-for-like.
Can a year have 53 weeks?
Yes — most years have 52 ISO weeks, but years where January 1st falls on a Thursday (or a leap year where it falls on Wednesday) get a 53rd week, since the ISO rule is anchored to the first Thursday rather than a fixed 52-week count.
Is my numbers uploaded anywhere?
No — Calculatrice de Numéro de Semaine runs entirely in your browser using JavaScript/WebAssembly. Your numbers is never sent to a server.