Week Number NowToday’s week number
What week is it?
ISO 8601 weeks start on Monday. Week 1 is the week containing the first Thursday of the year.
Calculated with your device’s time zone: local time.
Week ends in
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Day of year
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Days left in year
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Quarter
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Year progress: – · – weeks
Date → week number
Week number → dates
Weeks between two dates
Week numbers
All weeks of the year
How week numbers work
A week number tells you which week of the year a date falls in. Planners, factories, shipping companies, schools and software teams use week numbers because “week 43” is shorter and less ambiguous than a date range. There are two systems you will commonly meet in North America:
ISO 8601 weeks run Monday to Sunday. Week 1 is the week containing the first Thursday of January, so every week belongs entirely to one week-numbering year. This is the international standard and the default on this page.
US weeks run Sunday to Saturday, and week 1 is whichever week contains January 1. This is the default in many US calendars and in spreadsheet functions such as WEEKNUM(). Both systems usually agree for most of the year but can differ by one around New Year.
Frequently asked questions
Quick answers about week numbers, ISO 8601 and calendars.
What is the ISO 8601 week number?
ISO 8601 is the international standard for writing dates and times. Its week-numbering system starts every week on Monday, and week 1 of a year is the week that contains the year’s first Thursday (equivalently, the week that contains January 4). Weeks are numbered 01 to 52 or 53. It is the most widely used week system in Europe and in international business, and it is what this site shows by default.
How is the US week number different?
In the common US convention, weeks start on Sunday and week 1 is simply the week that contains January 1, even if it has only one day in the new year. Because of that, the last few days of December and the first few days of January can carry different numbers in each system. This matches the default of the WEEKNUM function in Excel and Google Sheets, while ISOWEEKNUM returns the ISO week. Use the ISO / US toggle at the top to switch.
Why do some years have 53 weeks instead of 52?
A year has 365 or 366 days, which is 52 weeks plus one or two extra days. Those extra days accumulate, so occasionally a year gets a 53rd ISO week. Under ISO 8601 a year has 53 weeks if it starts on a Thursday, or if it is a leap year that starts on a Wednesday. That happens in 71 out of every 400 years. Recent and upcoming 53-week ISO years are 2020, 2026, 2032, 2037, 2043 and 2048.
Does week 1 always contain January 1?
Not in the ISO system. If January 1 falls on a Friday, Saturday or Sunday, those days belong to the last week (52 or 53) of the previous year. For example, January 1, 2027 is a Friday, so it is part of ISO week 53 of 2026 (written 2026-W53). Likewise, December 29–31 can belong to week 1 of the next year. In the US system, January 1 is always in week 1.
How do I write a week number in ISO format?
The ISO 8601 format is YYYY-Www, for example 2026-W43. Add the day of the week (1 = Monday … 7 = Sunday) for a full week date: 2026-W43-5 is Friday of week 43. The compact form drops the hyphens: 2026W435. Note that YYYY is the ISO week-numbering year, which can differ from the calendar year for a few days around New Year.
Which time zone does this page use?
Your own. The current week, countdown and highlighted day are calculated entirely in your browser using your device’s clock and time zone, so someone in Vancouver and someone in Toronto each see the correct week for where they are, even around midnight on Sunday. If the result looks wrong, check your device’s date and time settings.
Who uses week numbers?
Week numbers are common in Europe (especially Germany, the Nordic countries and the Netherlands) for scheduling, school terms and holidays. Worldwide they are used in manufacturing, logistics, retail and finance for production planning, shipping schedules, payroll and sprint planning, and many companies run fiscal calendars built on weeks.
How many weeks are left in the year?
Look at the “Days left in year” card above, or open the full-year table, where every remaining week is listed with its dates. To count weeks between any two dates, use the week difference calculator.
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