ISO week numbers explained
By Dexaloop · Updated 27 October 2026 · Checked against the sources listed below
An ISO week runs Monday to Sunday, and week 1 of a year is the week that contains that year's first Thursday. Every year has 52 or 53 ISO weeks. Because the rule is about Thursday, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next. This is the system set by the international standard ISO 8601 and used widely in Europe for business planning.
The ISO 8601 rules in full
The ISO week system has only three rules:
- Weeks start on Monday. Monday is day 1 and Sunday is day 7.
- Week 1 is the week with the year's first Thursday. The Cambridge summary of ISO 8601 states that this "is equivalent to the week that contains the fourth day of January." In other words, week 1 is the first week with most of its days (four or more) in the new year.
- Weeks are numbered straight through. Every day belongs to exactly one week, and every week belongs to exactly one "week year". The week before week 1 is the last week, 52 or 53, of the previous year.
The standard also has a written form for week dates: the year, a W, the week, and optionally the day. Tuesday, December 31, 1996 is written 1997-W01-2, because that Tuesday falls in the first week of 1997.
Why Thursday?
Thursday is the middle day of a Monday to Sunday week. If a week's Thursday is in the new year, then at least four of its seven days are too. Picking Thursday as the test makes sure each week goes to the year that owns most of it. It also means no year ever has a stub week of one or two days at either end.
When does a year have week 53?
A normal year has 365 days, which is 52 weeks and 1 day. A leap year has 52 weeks and 2 days. Those spare days pile up until a year needs a 53rd week. Working through the rule above gives a simple test:
- A common year that starts on a Thursday has 53 weeks.
- A leap year that starts on a Wednesday or a Thursday has 53 weeks.
- Every other year has 52 weeks.
| Year | January 1 is a | ISO weeks | ISO week 1 starts |
|---|---|---|---|
| 2020 (leap) | Wednesday | 53 | Monday, December 30, 2019 |
| 2026 | Thursday | 53 | Monday, December 29, 2025 |
| 2027 | Friday | 52 | Monday, January 4, 2027 |
| 2028 (leap) | Saturday | 52 | Monday, January 3, 2028 |
The Cambridge summary notes that ISO week numbers always fall in the range 01 to 53, and that every year has a week 52.
Worked example: the turn of 2026 into 2027
2026 starts on a Thursday, so it has 53 weeks. Week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027.
- Thursday, December 31, 2026 is 2026-W53-4.
- Friday, January 1, 2027 is 2026-W53-5. It is in 2027 on the calendar, but in week 53 of the 2026 week year.
- Sunday, January 3, 2027 is 2026-W53-7.
- Monday, January 4, 2027 is 2027-W01-1. Week 1 of 2027 starts here, because this week holds Thursday, January 7, the first Thursday of 2027.
So a planner that says "week 1" for January 1, 2027 is not using ISO weeks.
How US week numbering differs
The United States has no single national week numbering rule, and several systems are in use. The two you meet most often are:
Sunday start, January 1 rule
Many US wall calendars and diaries start the week on Sunday and call the week that holds January 1 week 1. This is also the default in Excel. Microsoft describes its first system this way: "The week containing January 1 is the first week of the year, and is numbered week 1." Plain WEEKNUM(date) uses that rule with weeks starting on Sunday.
Under this rule, Friday, January 1, 2027 is week 1, and week 2 starts on Sunday, January 3. Week 1 can be as short as one day, and a year can have 53 or even 54 partial weeks.
CDC epidemiological weeks
US public health reporting uses MMWR weeks. The CDC fact sheet states that "The first day of any MMWR week is Sunday," and that week 1 "is the first week of the year that has at least four days in the calendar year." This is the ISO idea, but with Sunday weeks. MMWR and ISO week numbers often match, but they are not the same on every date.
| System | Week starts | Week 1 is the week that | Jan 1, 2027 is in |
|---|---|---|---|
| ISO 8601 | Monday | has the first Thursday | Week 53 of 2026 |
| Excel WEEKNUM default | Sunday | contains January 1 | Week 1 of 2027 |
| CDC MMWR | Sunday | has 4+ days in the new year | Last week of 2026 |
Getting it right in a spreadsheet
In Excel, ISOWEEKNUM(date) returns the ISO week. Microsoft's WEEKNUM page lists return type 21 as the ISO 8601 system, so WEEKNUM(date, 21) gives the same result. Remember that the week year can differ from the calendar year in early January and late December. If you build a code like 2027-W01 from YEAR(date) and the week number, January 1, 2027 becomes "2027-W53", which is wrong. Take the year from the Thursday of the same week instead.
Common mistakes
- Assuming January 1 is always week 1. In ISO weeks it is week 1 only when January 1 falls on Monday to Thursday.
- Mixing systems across teams. A European supplier's "week 1" and a US office's "week 1" can be different weeks. State the system or give dates.
- Using the calendar year for the week year. See the spreadsheet example above.
- Expecting 52 weeks every year. Payroll, sprint and reporting plans should allow for week 53.
Find the week number of any date: tools.dexaloop.com/week-number/. Free, no sign-up, and it runs on your device.
Questions
What is ISO week 1?
ISO week 1 is the Monday to Sunday week that contains the year's first Thursday. That is the same as the week that contains January 4. It can start as early as December 29 of the previous year.
Which years have 53 ISO weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2020 and 2026 are examples. Every other year has 52.
Why is January 1 in week 52 or 53?
If January 1 falls on a Friday, Saturday or Sunday, most of that week belongs to the old year, so ISO counts it as the old year's last week. For example, January 1 to 3, 2027 are in week 53 of 2026.
Does the US use ISO week numbers?
Not as a single national rule. Many US calendars and Excel's default WEEKNUM count weeks from Sunday with week 1 being the week that holds January 1. The CDC uses its own Sunday-start MMWR weeks. Always check which system a schedule uses.
How do I get the ISO week number in Excel?
Use ISOWEEKNUM(date), or WEEKNUM(date, 21). The plain WEEKNUM(date) uses a Sunday start and the January 1 rule, which gives different numbers in many weeks.
Sources
- Markus Kuhn, University of Cambridge: A summary of the international standard date and time notation (ISO 8601)
- CDC: MMWR Weeks fact sheet
- Microsoft Support: WEEKNUM function
- Microsoft Support: ISOWEEKNUM function
This guide is general information, not legal, tax or financial advice. Rules change; check the official source for your own case. Found a mistake? Email support@dexaloop.com.