What week is it?
Today, Sunday, October 18, 2026, is in ISO week 42 of 2026 (2026-W42). This week runs from Monday, October 12 to Sunday, October 18, 2026.
How ISO week numbers work
The ISO 8601 standard, used across Europe, in most business software and in many other countries, follows three rules:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year's first Thursday.
- A year has 52 or 53 weeks.
The rule for week 1 can be stated in several equivalent ways – use whichever is easiest to remember:
- It is the week containing January 4.
- It is the first week with at least four days in January.
- Its Monday is the Monday closest to January 1.
So 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. For example, January 1, 2027 is in week 53 of 2026, and December 30, 2024 was in week 1 of 2025.
Years with 53 weeks
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday – roughly 71 years in every 400. The next ones:
- 2026
- 2032
- 2037
- 2043
- 2048
- 2054
Writing a week date
| Format | Example | Meaning |
|---|---|---|
| Week | 2026-W42 | Week 42 of 2026 |
| Week and day | 2026-W42-7 | Sunday of week 42 (1 = Monday … 7 = Sunday) |
| Compact | 2026W427 | The same date without hyphens |
The year in a week date is the week-numbering year, which can differ from the calendar year for a few days around New Year.
Work out the week number by hand
\text{week} = \left\lfloor \frac{\text{day of the year} - \text{weekday} + 10}{7} \right\rfloor- Weekday: Monday = 1 … Sunday = 7;
\lfloor x \rfloormeans round down. - A result of 0 means the date is in the last week of the previous year.
- A result of 53 may really be week 1 of the next year – check whether the year has 53 weeks.
Day of the year: 291. Weekday: Sunday = 7.
\text{week} = \left\lfloor \frac{291 - 7 + 10}{7} \right\rfloor = \left\lfloor \frac{294}{7} \right\rfloor = 42The date is in week 42.
ISO weeks vs US weeks
| Rule | ISO 8601 | Common US system |
|---|---|---|
| First day of the week | Monday | Sunday |
| Week 1 | Week with the first Thursday | Week containing January 1 |
| Weeks in a year | 52 or 53 | 53, sometimes 54 (partial weeks) |
| Can Jan 1 be in last year's week? | Yes | No |
The two systems can differ by one week, so check which one your calendar, payroll or school uses. The result above shows both.
Week numbers in Excel or Google Sheets
| You want | Formula (date in A2) |
|---|---|
| ISO week number | =ISOWEEKNUM(A2) |
| US week number (weeks start Sunday) | =WEEKNUM(A2) |
| Monday of that week | =A2 - WEEKDAY(A2, 2) + 1 |
Frequently asked questions
What week number is it today?
Today is ISO week 42 of 2026. The calculator opens with today’s date, so press Calculate to see its first and last days.
How many weeks are in a year?
52 in most years and 53 in about one year in six, by the ISO standard. A year always has 52 full weeks plus one or two extra days.
Why can January 1 be in week 52 or 53?
ISO week 1 is the week with the year’s first Thursday. If January 1 falls on a Friday, Saturday or Sunday, it still belongs to the last week of the previous year.
Does the week start on Monday or Sunday?
In the ISO standard it starts on Monday. Many US calendars start it on Sunday, which can shift the week number by one.
What does 2026-W42 mean?
The 42nd week of the 2026 week-numbering year. Adding a digit, such as 2026-W42-3, names a day in that week (3 = Wednesday).