🗓️ Week Number

See today’s week number under both ISO 8601 (Monday start) and the U.S. Sunday-start system, check any date, and list every week of the year.

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ISO 8601 week (Monday start)
1
2026-W01 · Dec 29 – Jan 4, 2026
U.S. week (Sunday start)
1
Dec 28 – Jan 3, 2026

Thursday, January 1, 2026 is day 1 of 2026 — 364 days remain. ISO year 2026 has 53 weeks.

All ISO weeks in 2026
WeekMondaySunday
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

What Week Number Does

This page shows the current week number the moment it loads — in both of the systems people actually use — and lets you pick any other date to look up. Next to the week number you get the exact Monday-to-Sunday (or Sunday-to-Saturday) range, the day of the year, the days left in the year, and a table of every week in the year.

Two week-numbering systems are in wide use. ISO 8601, the international standard followed across Europe and in most business software, starts weeks on Monday and counts week 1 as the week containing the year’s first Thursday. The common U.S. system starts weeks on Sunday and counts the week containing January 1 as week 1. The two often differ by one, which is why a colleague abroad may quote a different week than your calendar.

How to Use Week Number

  1. Open the page to see today’s ISO and U.S. week numbers
  2. Pick any date to see its week number, week dates and day of the year
  3. Expand the list to see every ISO week of the year with its Monday and Sunday

Formula Used by Week Number

ISO 8601 week number

Week = ⌊(day of year − weekday + 10) ÷ 7⌋, with weekday 1 = Monday … 7 = Sunday

Result 0 or 53+
Belongs to the last week of the previous year, or week 1 of the next year

Worked example

Thursday, September 24, 2026.

  1. Day of year = 267, weekday = 4
  2. (267 − 4 + 10) ÷ 7 = 39.0

Result: ISO week 39 (2026-W39-4), running Monday Sep 21 to Sunday Sep 27.

Years With 53 ISO Weeks

A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year.

YearJanuary 1ISO weeks
2020Wednesday (leap year)53
2026Thursday53
2027Friday52
2032Thursday (leap year)53
2037Thursday53

Source: ISO week date — Wikipedia

How to Read Your Result

Early January and late December

Under ISO 8601, the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next. January 1, 2027 is a Friday, so it falls in 2026-W53. The page flags when the ISO year differs from the calendar year.

Spreadsheets

In Excel and Google Sheets, ISOWEEKNUM(date) returns the ISO week. WEEKNUM(date) defaults to the U.S. Sunday-start system; WEEKNUM(date, 21) returns the ISO week as well.

Limitations & Accuracy Notes

  • Week numbers are calculated for the date you pick, using your device’s local date for “today”.
  • Some countries and industries use other systems, such as weeks starting on Saturday or fiscal years that do not start in January.

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Frequently Asked Questions

What week of the year is it?
The page shows today’s week number as soon as it loads, in both systems. On September 24, 2026, for example, it is ISO week 39 (Monday, September 21 to Sunday, September 27) and U.S. week 39.
How are ISO week numbers calculated?
Under ISO 8601, weeks run Monday to Sunday and week 1 is the week that contains the year’s first Thursday — equivalently, the week containing January 4. Because of that rule, January 1–3 can belong to the last week of the previous year, and December 29–31 to week 1 of the next.
Why do my calendar and my spreadsheet show different week numbers?
They are probably using different systems. U.S. calendars and Excel’s default WEEKNUM start weeks on Sunday and count the week containing January 1 as week 1. ISO week numbers, used in Europe and by ISOWEEKNUM, start on Monday and use the first-Thursday rule. They often differ by one near the start of the year.
How many weeks are in a year?
ISO years have 52 or 53 weeks. A year has 53 when January 1 falls on a Thursday, or on a Wednesday in a leap year — 2026 is one, and the next is 2032. In the U.S. system the last few days of a year form a partial week 53 (and occasionally 54).
What is ISO week-date format?
It writes a date as year, week and weekday: 2026-W39-4 is Thursday of ISO week 39 in 2026. It is common in manufacturing, logistics and payroll because every week has exactly seven days and a fixed number.
What day of the year is it?
The result also shows the day of the year (1 to 365, or 366 in a leap year) and how many days remain — the “Julian day” number often printed on packaging and used in shipping.

References & Further Reading

By OnlineToolHubs Team • Updated September 2026