DocomintWeek Number Calculator
Find a date's ISO 8601 week number — the standard used in business and project planning.
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.
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.
How it works
- Enter any date
- Press Find Week Number
- The ISO 8601 week number and its associated ISO year appear instantly, computed on your device
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.
Is my numbers uploaded anywhere?
No — Week Number Calculator runs entirely in your browser using JavaScript/WebAssembly. Your numbers is never sent to a server.