Week Number Calculator

Convert any date into its ISO 8601 week number, US week number, and day of the year.

Inputs

Result

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General calculation reads

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Scientific calculator (Casio fx-991EX)
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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

Why are ISO and US week numbers sometimes off by one?+
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.
Can ISO weeks belong to a different year than the date?+
Yes — Jan 1 of a year sometimes falls in week 52 or 53 of the previous ISO year, and Dec 30/31 sometimes belongs to week 1 of the next ISO year. The calculator reports the correct ISO year alongside the week number.
How many weeks are in a year?+
52 or 53 in the ISO system. A 53-week year happens when Jan 1 is a Thursday, or in leap years when Jan 1 is a Wednesday.
What is day-of-year used for?+
It is a 1..365/366 ordinal that some logistics and broadcast schedules use ("Julian day" in those contexts). It is also handy for back-of-the-envelope progress calculations ("we are on day 187 of 365").
Is the day-of-week from this calculator using UTC?+
Yes — the date is interpreted as a UTC date so the week-day name is consistent with the input.

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