Find the ISO week number of any date, the US-style week and the day of the year. Runs on your device.
In the ISO 8601 system weeks start on Monday, and week 1 of a year is the week that contains the first Thursday of that year. That is the same as saying week 1 is the week that contains 4 January. The week-year is the year of that Thursday, which is why it can differ from the calendar year. 31 December 2020 is in ISO week 53 of 2020. 3 January 2021 is a Sunday, so it still belongs to week 53 of 2020 and is written 2020-W53. 30 December 2024 is a Monday and the Thursday of its week is 2 January 2025, so it is week 1 of 2025, written 2025-W01. A year has 52 or 53 ISO weeks. The page works this out from the Thursday of the date's week and shows the Monday to Sunday dates of that week.
The second answer follows the common US convention: weeks start on Sunday and week 1 is the week that contains 1 January. Numbers restart on 1 January, so the days before the first Sunday are in week 1, and the last few days of December can form a short week 53 instead of joining week 1 of the next year. This matches the default of the WEEKNUM function in spreadsheets. It is one convention among several, and calendars or payroll systems may use another, such as week 1 being the first full week. If a form asks for an ISO week, use the first answer.
The day of the year counts 1 January as day 1, and 31 December is day 365, or day 366 in a leap year. Days left in the year is the number of days after the chosen date up to and including 31 December, so on 31 December it is 0. The date is read as a calendar date, not a moment, so your time zone does not change the answer. Leave the date empty and the page uses today's date from your device. Years from 0001 to 9999 are accepted.
53. The ISO week-year is 2020, so it is written 2020-W53. The week runs from Monday 28 December 2020 to Sunday 3 January 2021, and 3 January 2021 is still in week 53 of 2020.
Because ISO week 1 is the week with the first Thursday of the year. The Thursday of the week of 30 December 2024 is 2 January 2025, so that week is week 1 of 2025, written 2025-W01.
They start the week on different days, Monday and Sunday, and they define week 1 differently. ISO week 1 contains the first Thursday, and the US-style week 1 contains 1 January. Near the start and end of a year they can differ by one week or more.
No. An ISO year has 52 or 53 weeks. It has 53 when 1 January is a Thursday, or a Wednesday in a leap year. 2020 had 53 weeks and 2026 has 53 as well.
Yes. An empty date means today, from your device's clock. Type a date to look up any other day.