Skip to content
Convertto

ISO Week Number Calculator

Find the ISO 8601 week number, quarter and day of year for any date.

To find an ISO week number, enter a date. ISO 8601 weeks begin on Monday and week 1 is the week containing the first Thursday of the year, which means the first days of January can belong to week 52 or 53 of the previous year.

Runs in your browser
Privacy
Runs entirely in your browser — nothing is uploaded
Cost
Free, unlimited, no sign-up

Frequently asked questions

Why is 1 January sometimes in week 53 of the previous year?

Under ISO 8601 a week belongs to whichever year contains its Thursday. If 1 January falls on a Friday, Saturday or Sunday, that week's Thursday was in December — so those days count as the last week of the old year.

How to use the iso week number calculator

  1. 1Enter or paste your date.
  2. 2The result appears immediately — copy or download it.

Embed this tool

Put the working iso week number calculator on your own site. It runs in your visitors' browsers exactly as it does here — free, no account, nothing uploaded.

Share this tool

Related tools

Day of Year CalculatorFind the ordinal day number, week number, quarter and how much of the year has elapsed.Age CalculatorWork out exact age in years, months and days — plus total days and the next birthday.Date Format ConverterReformat a list of dates into ISO, US, European, RFC or any custom pattern.Leap Year CheckerCheck whether a year is a leap year and see the next and previous ones, with the rule explained.Unix Timestamp ConverterConvert Unix timestamps to human dates and back, in seconds or milliseconds.Add or Subtract Days from a DateFind the date a set number of days, weeks, months or years before or after another.

Last updated

More date & time tools