Week Number Calculator
Convert any date into its ISO 8601 week number, US week number, and day of the year.
Result
Week number
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How to use this calculator
- Enter a date.
- Read the ISO and US week numbers, plus contextual info like day-of-year and quarter.
About this calculator
Different industries count weeks differently. ISO 8601 (used in most of Europe and by Linux/Unix tooling) starts each week on Monday and assigns week 1 to the week containing the first Thursday. US convention starts each week on Sunday and assigns week 1 to the week containing Jan 1. This calculator returns both, plus the day-of-year and day-of-week.
Frequently asked
They use different rules for what counts as week 1. ISO 8601 anchors on the first Thursday; US convention anchors on Jan 1. Around the new year the two systems can disagree by a week, especially when Jan 1 falls on a weekend.
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