Week Number Calculator
Determines the ISO calendar week number for any date.
Lilu Baba's Week Number Calculator finds the ISO-8601 week number for any date — the standard used in business planning, payroll and project schedules across most of the world.
How to use this calculator
- Enter any date.
- Its ISO-8601 calendar week number updates instantly.
ISO Week Number
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ISO Week Year
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FAQ
Week Number Calculator — frequently asked questions
A standard where week 1 is the week containing the year's first Thursday, and weeks run Monday to Sunday — used widely outside the US.
Yes — dates near the start or end of a calendar year can belong to a week counted in the adjacent year under ISO rules.