ISO Week Numbers Explained: How Week 1, Week 53 and 2026-W44-5 Work

ISO week 1 is the Monday-to-Sunday week that contains Jan. 4, according to the first-Thursday rule in the International Organization for Standardization (ISO) 8601 date standard. Weeks are numbered 01 to 53, and the week-year can differ from the calendar year: week 1 of 2026 began on Monday, Dec. 29, 2025. The table below lists the week 1 start date for every year from 2024 to 2035, a ready worked example is further down, and a one-page printable (below) keeps both.

What ISO week numbers are

ISO’s help center describes ISO 8601 as a standard that “provides a clear and unambiguous way to represent dates and times.” One part of it numbers whole weeks, so “week 44” names the same seven days for everyone who uses the system. A summary of the standard hosted by NASA gives the basics: possible ISO week numbers are 01 to 53, and a week starts with Monday (day 1) and ends with Sunday (day 7).

That Monday start separates ISO weeks from the Sunday-start grids on many wall calendars. Our Monday-start calendar guide covers the layout side of that choice, and the ISO standard itself has its own explainer: what ISO 8601 is.

How ISO week 1 is picked

The summary hosted by NASA says week 01 “is per definition the first week that has the Thursday in this year.” Google’s ISOWEEKNUM help page words it the same way: week 1 is the week containing the first Thursday of the year. Both rules land on the same Monday as “the week that contains Jan. 4.” We checked that equivalence for 2024 to 2035 with two independent date calculations, and the table uses it.

The consequence is that Jan. 1 does not have to be in week 1. When Jan. 1 is a Friday, Saturday or Sunday, it sits in the last week of the previous ISO year, and week 1 begins a few days later. When Jan. 1 is a Tuesday, Wednesday or Thursday, week 1 begins in the previous December.

Year	Jan. 1 is a	ISO week 1 starts (Monday)	Starts in December?	ISO weeks in the year
2024	Monday	Jan. 1, 2024	No	52
2025	Wednesday	Dec. 30, 2024	Yes	52
2026	Thursday	Dec. 29, 2025	Yes	53
2027	Friday	Jan. 4, 2027	No	52
2028	Saturday	Jan. 3, 2028	No	52
2029	Monday	Jan. 1, 2029	No	52
2030	Tuesday	Dec. 31, 2029	Yes	52
2031	Wednesday	Dec. 30, 2030	Yes	52
2032	Thursday	Dec. 29, 2031	Yes	53
2033	Saturday	Jan. 3, 2033	No	52
2034	Sunday	Jan. 2, 2034	No	52
2035	Monday	Jan. 1, 2035	No	52

Fresh Calendars computation from the week 1 rule above; weekdays and counts checked with two independent methods.

Our year-by-year list of 2027 week boundaries is in the 2027 week numbers printable.

How to read an ISO week date: 2026-W44-5

An ISO week date strings together three numbers. The summary hosted by NASA shows the pattern with the example 1997-W01-2. Applied to a date from this story’s publication day:

Part	Value in 2026-W44-5	Meaning
2026	ISO week-year	The year that owns the week, which can differ from the calendar year
W44	Week number	The 44th Monday-to-Sunday week counted from week 1
5	Weekday	1 is Monday and 7 is Sunday, so 5 is Friday

By our calculation, 2026-W44-5 is Friday, Oct. 30, 2026. Week 44 runs from Monday, Oct. 26 to Sunday, Nov. 1, 2026. The same string is not a calendar date in the YYYY-MM-DD form that ISO’s help center describes, so a system that expects 2026-10-30 will reject 2026-W44-5.

The week-year matters at the edges of the year. Jan. 1, 2027 is a Friday, which puts it in ISO week 53 of 2026: its week date is 2026-W53-5, even though a spreadsheet year function (YEAR) returns 2027. Anyone building a report by week number should take the week-year from the same ISO calculation as the week number, not from the calendar year.

ISO weeks and Sunday-start week numbers

Spreadsheet week numbers are not all ISO. Microsoft’s WEEKNUM function takes a return type: 1 or omitted means the week begins on Sunday under System 1, 2 means it begins on Monday under System 1, and 21 means it begins on Monday under System 2. System 1 counts the week that contains Jan. 1 as week 1; System 2 is the ISO method. Microsoft Outlook’s default first day of the week is Sunday, and calendar apps such as Google Calendar and Apple Calendar let you choose the start day, so two people can see different week labels for one date.

Here is the same set of dates under the three schemes, computed by Fresh Calendars from those rules.

Date	ISO week date	WEEKNUM, Sunday start (System 1)	WEEKNUM, Monday start (System 1)
Fri., Oct. 30, 2026	2026-W44-5	44	44
Thu., Dec. 31, 2026	2026-W53-4	1	1
Fri., Jan. 1, 2027	2026-W53-5	1	1
Sun., Jan. 3, 2027	2026-W53-7	2	1
Mon., Jan. 4, 2027	2027-W01-1	2	2

For late October the three agree. At the turn of 2027 they split: the ISO week that began on Monday, Dec. 28, 2026 is week 53, while System 1 already calls that week 1 because it contains Jan. 1. If a contract, a payroll calendar or a shared planner says “week 1,” ask which system it uses. Our printable weekly calendar and Monday-start printable calendar show the week grid without numbering, so you can add the numbers from the system your team has chosen.

How to find the ISO week number of a date

Microsoft’s ISOWEEKNUM function returns the ISO week number of the year for a given date, with the syntax ISOWEEKNUM(date). Google’s help page documents the same function and states that weeks begin on Monday and end on Sunday. For a date in cell A1, =ISOWEEKNUM(A1) returns 44 for Oct. 30, 2026 and 53 for Jan. 1, 2027.

To confirm a result by hand:

1. Write down the date’s weekday and the Monday of its week.
2. Find the Monday of week 1 for the year in the table above.
3. Count the whole weeks from that Monday, starting at 1.
4. If the date falls before that year’s week 1 Monday, it belongs to the last week of the previous ISO year.

For Oct. 30, 2026, the Monday is Oct. 26. Week 1 began on Dec. 29, 2025, and 43 whole weeks later is Oct. 26, 2026, so the week number is 44.

Why some ISO years have 53 weeks

A common year has 365 days and a leap year 366, per NASA’s calendar reference. Since 365 is 52 x 7 + 1 and 366 is 52 x 7 + 2, one or two days are left over each year, and over the years they add up to a whole extra week. By our computation an ISO year has 53 weeks when Jan. 1 is a Thursday, or when it is a leap year that starts on a Wednesday. Equivalently, Dec. 28 always falls in the last ISO week of its year, because it is seven days before Jan. 4.

The 53-week ISO years from 2000 to 2050 are 2004, 2009, 2015, 2020, 2026, 2032, 2037, 2043 and 2048, by our calculation. The full rule, with more years, is in why some years have 53 weeks.

Printable: ISO week 1 start dates, 2024 to 2035

The printable below has the twelve week 1 Mondays from the table plus the reading rules in four lines, on one US Letter page with an ink-saver version. It is built from the same computation as this story.