An ISO week number identifies each week of the year under the international standard ISO 8601. Weeks run Monday through Sunday, and week 1 is the week that contains the year’s first Thursday (equivalently, the week containing January 4). Most years have 52 ISO weeks; some have 53. 2026 has 53, because it began on a Thursday. To find a date’s week number, use the formula below or the week number calculator.
The rules of ISO 8601 weeks
- Every week starts on Monday and ends on Sunday.
- Week 1 is the first week with at least four days in the new year, which is the same as the week containing the first Thursday, or the week containing January 4.
- A year has 53 weeks if it starts on a Thursday, or if it is a leap year that starts on a Wednesday. Otherwise it has 52.
- Days at the very start or end of January or December can belong to the previous or next ISO year.
Dates are written as year, week and weekday: 2026-W41-7 is the 7th day (Sunday) of week 41 of ISO year 2026, which is October 11, 2026.
The formula
Here Day of year is 1 for January 1, and Weekday is 1 for Monday through 7 for Sunday. Then check the edges:
- If the result is 0, the date belongs to the last week of the previous year (week 52 or 53).
- If the result is 53 and the year does not have 53 weeks, the date belongs to week 1 of the next year.
Example: October 11, 2026
- Day of year: January through September contain 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 = 273 days, so October 11 is day 284.
- Weekday: Sunday = 7.
- Week: ⌊(284 − 7 + 10) ÷ 7⌋ = ⌊287 ÷ 7⌋ = 41.
Example: January 1, 2027
- Day of year: 1. Weekday: Friday = 5.
- Week: ⌊(1 − 5 + 10) ÷ 7⌋ = ⌊6 ÷ 7⌋ = 0.
- A result of 0 means the last week of 2026. Since 2026 has 53 weeks, January 1, 2027 is in 2026-W53.
2026 and 2027 week boundaries
| ISO week | Monday | Sunday |
|---|---|---|
| 2026-W01 | December 29, 2025 | January 4, 2026 |
| 2026-W02 | January 5, 2026 | January 11, 2026 |
| 2026-W41 | October 5, 2026 | October 11, 2026 |
| 2026-W53 | December 28, 2026 | January 3, 2027 |
| 2027-W01 | January 4, 2027 | January 10, 2027 |
So December 29–31, 2025 are in ISO year 2026, and January 1–3, 2027 are still in ISO year 2026. When you file reports by ISO week, use the ISO year, not the calendar year, or those days will land in the wrong year.
Which years have 53 weeks?
| Years with 53 ISO weeks (2015–2040) | Why |
|---|---|
| 2015 | Starts on Thursday |
| 2020 | Leap year starting on Wednesday |
| 2026 | Starts on Thursday |
| 2032 | Leap year starting on Thursday |
| 2037 | Starts on Thursday |
About 71 of every 400 years have 53 weeks, roughly one every five to six years. Businesses that budget by week need to plan for the extra week: payroll, rent or subscription costs computed “per week” will be about 1.9% higher in a 53-week year.
ISO weeks vs. US week numbers
Two other conventions are common in the United States:
| System | Week starts | Week 1 is | Example: October 11, 2026 |
|---|---|---|---|
| ISO 8601 | Monday | Week with the first Thursday | Week 41 |
| US (Excel WEEKNUM default) | Sunday | Week containing January 1 | Week 42 |
In the US system, week 1 of 2026 is Thursday, January 1 to Saturday, January 3, a partial week of only three days, and Sunday, January 4 starts week 2. That is why US week numbers often run one ahead of ISO numbers and why the US system can have 53 or even 54 numbered weeks. Retailers often use yet another scheme, a 52/53-week fiscal calendar divided into 4-5-4 week quarters.
Whenever you share a week number with someone else, say which system you mean.
Why Thursday?
The first-Thursday rule is a majority rule in disguise. A Monday-to-Sunday week that contains the year’s first Thursday has at least four of its seven days in the new year, so the week “belongs” to the year that holds most of it. If January 1 falls on Friday, Saturday or Sunday, only one to three days of that week are in January, so the week is counted as the last week of the old year instead. This guarantees that every ISO week is a full seven days and that no week is ever split between two week-numbering years.
Week numbers in spreadsheets and code
| Tool | ISO week | ISO year |
|---|---|---|
| Excel / Google Sheets | =ISOWEEKNUM(A1) |
=YEAR(A1-WEEKDAY(A1,2)+4) |
| Excel / Google Sheets (alternative) | =WEEKNUM(A1,21) |
— |
| Python | date.isocalendar().week |
date.isocalendar().year |
| SQL (PostgreSQL) | EXTRACT(WEEK FROM d) |
EXTRACT(ISOYEAR FROM d) |
The ISO-year formula works by moving to the Thursday of the date’s week; that Thursday always falls in the correct ISO year.
Finding the dates for a given week number
To go from a week number to dates, find the Monday of week 1, then add 7 days for each later week:
The Monday of week 1 is the Monday on or before January 4. For 2026, January 4 is a Sunday, so the Monday of week 1 is December 29, 2025, and week 41 starts 280 days later, on October 5, 2026. The date calculator adds the days, and the day of the week calculator confirms the weekday; how to find the day of the week for any date shows how to do that step by hand.
Where ISO weeks are used
- Business planning and reporting in Europe and in many global companies.
- Manufacturing and logistics, where production schedules and shipping weeks are numbered.
- Software and data, because ISO weeks never split across a week boundary and every week has exactly 7 days.
- Payroll and shift scheduling, where a 53-week year can mean an extra pay period. See how many work hours are in a year.
For a month-by-month view of the year, the printable calendar is handy.
Frequently asked questions
What week number is it?
Under ISO 8601, take the date's day of the year, subtract its weekday number (Monday = 1 to Sunday = 7), add 10 and divide by 7, dropping the remainder. October 11, 2026 is day 284 and a Sunday: (284 − 7 + 10) ÷ 7 = 41, so it is in ISO week 41.
Does 2026 have 53 weeks?
Yes, in the ISO system. January 1, 2026 is a Thursday, and any year that starts on a Thursday has 53 ISO weeks. Week 53 of 2026 runs from Monday, December 28, 2026 to Sunday, January 3, 2027.
When does ISO week 1 of 2026 start?
On Monday, December 29, 2025. Week 1 is the week containing the year's first Thursday (January 1, 2026), so it includes three days of December 2025.
Why is my US calendar's week number different?
Many US calendars and Excel's default WEEKNUM start weeks on Sunday and call the week containing January 1 week 1. That system often gives a number one higher than ISO, and it can produce a partial week 1 and a week 53 or 54.
What Excel formula returns the ISO week number?
=ISOWEEKNUM(A1) returns the ISO 8601 week number for the date in A1. =WEEKNUM(A1,21) gives the same result. Plain =WEEKNUM(A1) uses the Sunday-start US system.