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Date & time

ISO week number calculator

Find the ISO week number, its week-year and the Monday of that week.

Free & no registration Calculated in your browser
01

Your inputs

The starting values are an example.

Decimals: 12.5 · Thousands: 1,234.56. Follow the field instructions for lists.

02

Your result

Enter your values and select Calculate.

Your calculation will appear here.

Your result will include the relevant values and units.

Understand the calculation

Formula and example

How it is calculated

ISO weeks begin on Monday. Week 1 contains 4 January.

Worked example

1 January 2021 falls in week 53 of ISO week-year 2020.

Things to check

  • Around New Year, the ISO week-year may differ from the calendar year.