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Week Number Calculator

ISO 8601 week number for any date.Date & Time Tools

Week Number Calculator

ISO 8601 week number for any date: weeks start on Monday and week 1 is the one containing the first Thursday.

About this tool

ISO weeks start on Monday and week 1 contains the first Thursday — which is why dates near New Year can belong to week 1 of the following year. This tool reports both correctly.

How to use the Week Number Calculator tool

  1. Pick a date. Any calendar date.
  2. Read the week. The ISO week number and the week year.
  3. Trust the edge cases. Year-boundary dates are the whole point of the tool.

Supported formats

Input

  • Plain text or file input

Output

  • Converted or analysed output

Inputs are validated before anything is processed. Invalid values and unsupported files are rejected with a clear error instead of failing silently.

Frequently asked questions

Why does Dec 30 say week 1?

ISO rules: that date belongs to week 1 of the next year, and the week year says so.

Do all countries use this system?

Most do; the US convention of weeks starting Sunday differs.

Is the week year the calendar year?

Usually, but not always at year boundaries — both are shown.

Privacy

Everything you enter stays in this browser tab. There is no upload endpoint for tool inputs, no account, and no server-side copy of your text or files. Results are produced locally and disappear when you close the page.

No account is required and no file is ever uploaded. See the privacy policy for details about analytics and the limited data this site stores.

Known limitations

  • No US-style week numbering mode.

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