Week Number
What week is it today? Find the ISO week number of any date, or the dates of any week, and see the whole year with week numbers.
Week numbers
| Week | Start | End |
|---|
How to find a week number
- The box at the top shows today’s week number, the Monday-to-Sunday range of the week and how many weeks are left in the year.
- To find the week of another day, pick it in the Date field. The week number, its seven days, the day of the year and the week of the month appear below.
- To go the other way, type a year and week number on the right, or step with the arrows. The result shows the Monday and Sunday of that week.
- Scroll down for a calendar of the whole year with a week number on every row, or switch to the list and copy every week’s start and end date as CSV. Use the buttons at the top to switch between ISO, US and Saturday-start numbering.
How ISO week numbers work
Most of Europe, Turkey and international business number weeks with the ISO 8601 standard. Its rules are short: a week always starts on Monday and ends on Sunday, and week 1 is the week that contains the year’s first Thursday. Put another way, week 1 is the week with 4 January in it, or the first week that has at least four days in the new year. Every week therefore belongs to exactly one year, and there are no half weeks.
Week-year: when 1 January is in week 52 or 53
Because weeks are never split, 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. ISO calls the year a week belongs to its week-year. For example, Friday 1 January 2027 is in week 53 of 2026, while Monday 29 December 2025 already starts week 1 of 2026. The tool tells you whenever the week-year differs from the calendar year. In ISO notation a week is written as 2026-W44 and a single day as 2026-W44-6, where the last digit is the weekday from 1 (Monday) to 7 (Sunday).
52 or 53 weeks?
An ISO year has 52 weeks, or 53 when it starts on a Thursday, or is a leap year that starts on a Wednesday. That happens about every five to six years: 2026 has 53 weeks, and the next ones are 2032 and 2037. Payroll, production plans and school timetables that count in weeks need to account for this extra week.
US and Middle Eastern week numbers
The United States, Canada and Japan usually start the week on Sunday and call the week that contains 1 January week 1, even if it has only one day of the new year. Several Middle Eastern countries use the same rule with weeks that start on Saturday. These systems can give a different number from ISO for the same day, typically one higher, and in this tool the last days of December move into week 1 of the next year, as in a printed calendar. Excel’s WEEKNUM function uses the same start but keeps counting, so it can return week 53 or even 54 for those December days. When you share a week number with someone abroad, say which system you mean, or simply give the dates. All calculations on this page run in your browser.
Week numbers in Excel and code
How to get the ISO week number of a date d, and the Monday of week w in year y.
| Tool | ISO week of a date | Monday of week w |
|---|---|---|
| Excel / Google Sheets | =ISOWEEKNUM(A1) | =DATE(y,1,4)-WEEKDAY(DATE(y,1,4),3)+7*(w-1) |
| Python | d.isocalendar().week | date.fromisocalendar(y, w, 1) |
| PHP | date('W', $ts) | (new DateTime())->setISODate($y, $w) |
| Java | d.get(IsoFields.WEEK_OF_WEEK_BASED_YEAR) | LocalDate.of(y, 1, 4).with(IsoFields.WEEK_OF_WEEK_BASED_YEAR, w).with(DayOfWeek.MONDAY) |
| C# | ISOWeek.GetWeekOfYear(d) | ISOWeek.ToDateTime(y, w, DayOfWeek.Monday) |
| JavaScript | Temporal.PlainDate.from(d).weekOfYear | — |
| MySQL | WEEK(d, 3) | STR_TO_DATE(CONCAT(y, ' ', w, ' Monday'), '%x %v %W') |
| PostgreSQL | EXTRACT(WEEK FROM d) | to_date('2026-44-1', 'IYYY-IW-ID') |
| SQL Server | DATEPART(ISO_WEEK, d) | — |
| Bash (GNU date) | date +%V | — |
Excel’s WEEKNUM(A1) without a second argument gives US-style numbers (Sunday start, week 1 contains 1 January). WEEKNUM(A1,21) is the same as ISOWEEKNUM. Java needs java.time.temporal.IsoFields; C# needs System.Globalization.
Frequently asked questions
What week number is it today?
Today’s week number is shown at the top of the page and updates by itself at midnight. On Saturday 31 October 2026 it is ISO week 44, which runs from Monday 26 October to Sunday 1 November 2026.
How is the ISO week number calculated?
ISO weeks start on Monday. Week 1 is the week that contains the first Thursday of the year, which is the same as the week that contains 4 January. Each following week adds one, so most days are in week 1 to 52, and some years also have a week 53.
Which years have 53 weeks?
An ISO year has 53 weeks when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday. 2020 and 2026 have 53 weeks; the next ones are 2032, 2037, 2043 and 2048. All other years have 52.
Why are ISO and US week numbers different?
The US system starts weeks on Sunday and counts the week with 1 January as week 1, even if only one day of it is in January. ISO starts on Monday and needs at least four days in the new year. For many dates the US number is one higher than the ISO number.
How do I get the week number in Excel or Google Sheets?
Use =ISOWEEKNUM(A1) for ISO weeks, as used in Europe and Turkey. =WEEKNUM(A1) gives US-style weeks that start on Sunday. To find the Monday of week w in year y, use =DATE(y,1,4)-WEEKDAY(DATE(y,1,4),3)+7*(w-1).