Week Number Calculator

See the ISO week number of any date with the week’s Monday-to-Sunday range — or look a week number up in reverse.

Mode
Results
ISO week
Week starts (Monday)
Week ends (Sunday)
Weeks in that year
Pick a date — its ISO week updates instantly.

Every lookup runs in your browser. The dates you pick are never uploaded to BroBroGo.

FAQ

How is week 1 of a year decided?

ISO weeks run Monday to Sunday, and week 1 is the week containing the year’s first Thursday — equivalently, the week that includes January 4. This is the numbering used by most European calendars, planners and the ISOWEEK function in spreadsheets.

Why does a late-December date show next year’s week 1?

A week that straddles New Year belongs entirely to the year containing its Thursday. December 29–31 can therefore fall in week 1 of the next year, and January 1–3 in week 52 or 53 of the previous one — the status bar points this out whenever it happens.

Does every year have 52 weeks?

Most years have 52 ISO weeks, but a year that starts on a Thursday — or a leap year that starts on a Wednesday — has 53, like 2026. That’s why calendar apps sometimes show a week 53.

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