ISO Week Numbers: How to Find the Week Number of Any Date

As of Friday, October 9, 2026, it is ISO week 41, which runs from Monday, October 5 to Sunday, October 11. ISO weeks come from ISO 8601, the International Organization for Standardization's date standard. The rule is short but has odd effects around New Year, and not every spreadsheet or calendar uses it. We computed every number below with a script, checking the week calculation against Python's isocalendar() for every date from 1583 to 9999.

What the ISO week rule says#

ISO 8601 sets out how to write dates and times (Unix Time, ISO 8601 and UTC: How Computers Store Time covers the notation) and also defines week numbers. Its rule has four parts:

• A week runs from Monday (day 1) to Sunday (day 7).
• A week belongs to the year that holds its Thursday. That year is the week-year (also called the ISO year), and near New Year it can differ from the calendar year.
• Week 1 is the week with the year's first Thursday. Thursday is the middle day of the week, so this is also the first week with at least four days in the new year, and the week that contains January 4.
• A week-year has 52 or 53 whole weeks.

The Python documentation states the Thursday rule, the Linux manual gives the four-day version, and a write-up from Utrecht University ties both to January 4. Week 1 can therefore start as early as December 29 or as late as January 4: Monday, December 29, 2025, for 2026, Monday, January 4, 2027, for 2027.

Together, week-year, week and weekday form a week date: 2026-W41-5 is Friday (day 5) of week 41 of 2026.

How to find the week number of any date#

Four steps, no software needed.

1. Note the weekday, counting Monday as 1 and Sunday as 7.
2. Move to the Thursday of the same week. Monday, Tuesday and Wednesday move forward 3, 2 and 1 days; Friday, Saturday and Sunday move back 1, 2 and 3.
3. The year of that Thursday is the week-year.
4. Divide the Thursday's day of the year (January 1 is day 1) by 7 and round up. That is the week number.

The last two rows show why step 3 matters: the Thursday, not your starting date, decides the year. You can also count from week 1. For 2026 it began on Monday, December 29, 2025, and October 9, 2026, is 284 days later, which is 40 whole weeks and 4 days, so week 41.

When the week-year is not the calendar year#

From January 4 to December 28 the week-year always matches the calendar year (checked for 1583 to 9999). Only December 29 to January 3 can land in a different week-year, and never more than three days of one calendar year do. Which days move depends on the weekday of January 1:

Why some years have 53 weeks#

Because each week belongs to the year of its Thursday, a week-year has as many weeks as its calendar year has Thursdays. A common (non-leap) year is 52 weeks plus one day, so the weekday it starts on occurs 53 times. A leap year is 52 weeks plus two days, so both its first and last weekdays do. A year therefore has 53 weeks when it starts or ends on a Thursday, a test the Utrecht write-up also gives.

From 2020 to 2040 it happens in four years:

A 53-week year does not lengthen the calendar year: 2026 still has 365 days. It is the week-year that is longer: 371 days instead of 364. The 400-year Gregorian cycle (Leap Years Explained: Why February Sometimes Has 29 Days) is 146,097 days, exactly 20,871 weeks, and 71 of its years have 53 weeks, five, six or occasionally seven years apart.

Why Excel and US-style calendars can disagree#

Week-numbering schemes differ in two settings: the day the week starts, and how many days of the new year week 1 must contain. Unicode's locale data standard calls them firstDay and minDays, and gives Monday with 4 as the pair that reflects ISO 8601 and many national standards. Java ships two ready-made pairs, WeekFields.ISO and WeekFields.SUNDAY_START, which use 4 days with Monday and 1 day with Sunday.

The middle column is Excel's default WEEKNUM, which Microsoft calls System 1; Teamup's calendar app calls it its US system. The right column is the CDC's MMWR (Morbidity and Mortality Weekly Report) week for US disease reporting: it also starts on Sunday but needs four days, so even in the US there is more than one scheme. We computed the last two columns from the published rules and did not run Excel.

Because week 1 can hold just one or two days of the new year, a Sunday-start count can run a week ahead for the whole year. For dates from January 1 to December 31, 2026, it matches ISO on every day except the 52 Sundays, which it numbers one higher; for 2027 it differs on all 365 days. In Excel, ISOWEEKNUM(date) or WEEKNUM(date, 21) returns the ISO number.

Where week numbers are used#

Week numbers are common in parts of Europe. The University of Gävle in Sweden tells students that Swedes use week numbers to refer to particular weeks, and Eurostat's notes for the EU Labour Force Survey define their reference week by the ISO 8601 rules. Because other systems number weeks differently, say "ISO week 41" or give the Monday date when you share one.

Why the week number depends on your time zone#

A new week starts at midnight local time, so the number depends on where you are. At 20:00 on Sunday, October 11, 2026, in New York (UTC-4), it is already Monday, October 12 in London (01:00), India (05:30) and Sydney (11:00). New York is still in week 41; those places are in week 42. The time zone converter and How to Convert Time Between Time Zones, Step by Step (With Examples) help with moments like this.

Frequently asked questions

Sources and further reading

1. datetime — Basic date and time types — Python Software Foundation
2. Mathematics of the ISO 8601 Calendar - The ISO Calendar — Utrecht University
3. The Mathematics of the ISO 8601 Calendar - Long and Short Years — Utrecht University
4. WEEKNUM function — Microsoft Support
5. Unicode Locale Data Markup Language (LDML) Part 4: Dates — Unicode Consortium
6. MMWR Weeks - NNDSS case definitions - CDC — Centers for Disease Control and Prevention
7. EU Labour Force Survey explanatory notes (January 14, 2026 version) — Eurostat