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Week Number

ISO 8601 weeks start Monday.

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How Week Number works

ISO 8601 weeks start Monday. Week 1 contains 4 January; the week-year may differ from the calendar year.

Try an example

2021-01-01 → 2020-W53.

Good to know

Date-only calculations use the Gregorian calendar and UTC day boundaries, so daylight-saving changes do not alter day counts. Years 0001–9999.

Your privacy

Processing happens on this device for this tool. We do not upload your input, put it in share links or save it to your favourites. Closing this tab discards the working input; downloaded results remain wherever you save them.

When this tool is useful

Use Week Number when you need a quick, local result and want the rule or limitation visible alongside it.

How to use it

  1. Enter the input using the labels shown.
  2. Run the operation and review the result.
  3. Download or copy the result only after checking it.

Checks before you rely on the result

  • Date-only calculations use the Gregorian calendar and UTC day boundaries, so daylight-saving changes do not alter day counts. Years 0001–9999.
  • Keep the original input when the operation creates a new copy.
  • Use the related tools below when the next step needs a different format.

This page is maintained with the tool implementation and its deterministic tests. If the result looks wrong, report the issue with a non-sensitive example.

Read the Week Number instructions

A useful next step

Date Difference
Add / Subtract Days
Business Days Calculator
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