Week Number CalculatorWhat week of the year is it? ISO 8601 and US week numbers

Find the week number for any date under the ISO 8601 standard and the US Sunday-start system, see the Monday-to-Sunday dates of any week, and browse every week of the year.

  • ISO 8601 exact
  • US WEEKNUM too
  • Week → dates lookup
  • 53-week years handled
Manzoor Ahmad
Designed, developed & tested by Manzoor Ahmad, calculator developer
Last reviewed
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Week number calculator

ISO week number

Week 45

—ISO week date
—US week (Sun-start)
—weeks in ISO year
—weeks left this year

…or find the dates of a week

 

All weeks of the year

Tap a week to see its Monday-to-Sunday dates. The highlighted week contains your date.

What week of the year is it?

Under the ISO 8601 standard used in Europe and most business software, weeks start on Monday, and week 1 is the week containing the year’s first Thursday. November 3, 2026 falls in week 45 (Monday, November 2 to Sunday, November 8). 2026 has 53 ISO weeks because it starts on a Thursday. US calendars often count differently: weeks start on Sunday and week 1 is the week containing January 1, so the US number can be one higher.

ISO vs US week numbering

RuleISO 8601 (international)US / Excel WEEKNUM default
Week starts onMondaySunday
Week 1 isThe week with the year’s first Thursday (equivalently, the week containing January 4)The week containing January 1
Weeks per year52 or 5353 or 54 (partial weeks at the ends)
Excel / Google Sheets=ISOWEEKNUM(A1)=WEEKNUM(A1)
Written as2026-W45-2 (year-week-day)“Week 45”

53-week years (2020–2050): 2020, 2026, 2032, 2037, 2043, 2048. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year.

ISO week numbers for 2026

WeekStarts (Monday)Ends (Sunday)
W01Dec 29, 2025Jan 4, 2026
W02Jan 5, 2026Jan 11, 2026
W03Jan 12, 2026Jan 18, 2026
W04Jan 19, 2026Jan 25, 2026
W05Jan 26, 2026Feb 1, 2026
W06Feb 2, 2026Feb 8, 2026
W07Feb 9, 2026Feb 15, 2026
W08Feb 16, 2026Feb 22, 2026
W09Feb 23, 2026Mar 1, 2026
W10Mar 2, 2026Mar 8, 2026
W11Mar 9, 2026Mar 15, 2026
W12Mar 16, 2026Mar 22, 2026
W13Mar 23, 2026Mar 29, 2026
W14Mar 30, 2026Apr 5, 2026
W15Apr 6, 2026Apr 12, 2026
W16Apr 13, 2026Apr 19, 2026
W17Apr 20, 2026Apr 26, 2026
W18Apr 27, 2026May 3, 2026
W19May 4, 2026May 10, 2026
W20May 11, 2026May 17, 2026
W21May 18, 2026May 24, 2026
W22May 25, 2026May 31, 2026
W23Jun 1, 2026Jun 7, 2026
W24Jun 8, 2026Jun 14, 2026
W25Jun 15, 2026Jun 21, 2026
W26Jun 22, 2026Jun 28, 2026
W27Jun 29, 2026Jul 5, 2026
W28Jul 6, 2026Jul 12, 2026
W29Jul 13, 2026Jul 19, 2026
W30Jul 20, 2026Jul 26, 2026
W31Jul 27, 2026Aug 2, 2026
W32Aug 3, 2026Aug 9, 2026
W33Aug 10, 2026Aug 16, 2026
W34Aug 17, 2026Aug 23, 2026
W35Aug 24, 2026Aug 30, 2026
W36Aug 31, 2026Sep 6, 2026
W37Sep 7, 2026Sep 13, 2026
W38Sep 14, 2026Sep 20, 2026
W39Sep 21, 2026Sep 27, 2026
W40Sep 28, 2026Oct 4, 2026
W41Oct 5, 2026Oct 11, 2026
W42Oct 12, 2026Oct 18, 2026
W43Oct 19, 2026Oct 25, 2026
W44Oct 26, 2026Nov 1, 2026
W45Nov 2, 2026Nov 8, 2026
W46Nov 9, 2026Nov 15, 2026
W47Nov 16, 2026Nov 22, 2026
W48Nov 23, 2026Nov 29, 2026
W49Nov 30, 2026Dec 6, 2026
W50Dec 7, 2026Dec 13, 2026
W51Dec 14, 2026Dec 20, 2026
W52Dec 21, 2026Dec 27, 2026
W53Dec 28, 2026Jan 3, 2027

Related calculators

Planning by week? These calendar tools fit alongside:

Day of the Week CalculatorFind the weekday of any date, with the Doomsday mental-math method.Open Day of the Week Calculator →Business Days CalculatorCount the working days in a week, sprint or month, skipping holidays.Open Business Days Calculator →Date CalculatorAdd a number of weeks to a date, such as ‘+12 weeks’ or ‘+40 weeks’.Open Date Calculator →Days Until CalculatorCount down to the end of the quarter or any target week.Open Days Until Calculator →

More Date & Countdown calculators in this cluster:

Manzoor Ahmad

Manzoor Ahmad

Calculator Developer · Founder of DexoCalc

I’m a pharmacist turned web developer with 13+ years of experience building web tools. Week numbers trip people up because the ISO and US systems disagree. This calculator shows both side by side. I verified the ISO results against Python’s isocalendar for every week from 2020 to 2050.

Published · Last reviewed · Author profile · Report an issue

Week numbers: frequently asked questions

The calculator opens on today’s date and shows the ISO week number, its Monday-to-Sunday dates and the US-style week number too.

2026 has 53 ISO weeks because January 1, 2026 is a Thursday. Week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027.

Under ISO 8601, a week belongs to the year that holds its Thursday. If January 1 falls on a Friday, Saturday or Sunday, it is part of the last week of the previous year. January 1, 2027 is in week 53 of 2026.

It means year 2026, week 45, day 2 (Tuesday, since Monday is day 1). It’s the ISO 8601 way to write a date by week.

Many US calendars and Excel’s WEEKNUM start weeks on Sunday and call the week containing January 1 ‘week 1’. That can make the US number one higher than the ISO number. Both appear in the results.

Enter a year and a week number under ‘find the dates of a week’, or tap any week in the grid.

Sources & references

  1. ISO 8601: Date and time format, International Organization for Standardization
  2. ISOWEEKNUM function, Microsoft Support
  3. WEEKNUM function, Microsoft Support

ISO weeks follow ISO 8601. Organizations may use their own fiscal-week calendars.

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