Date → week number
Week number → dates
All weeks of the year
ISO 8601: weeks start on Monday, and week 1 is the week with the first Thursday of the year. US: weeks start on Sunday, and week 1 is the week with 1 January. Your choice is saved in this browser.
About the week number tool
What week is it? This page shows today's week number in big letters, with the Monday to Sunday dates of this week. Below it you can look up the week of any date, find the dates of any week, and see a table of all weeks of a year.
Many countries plan work, school and deliveries by week. In Germany people say Kalenderwoche (KW 46), in the Netherlands weeknummer (week 46), and in Sweden vecka. They all use the ISO 8601 week, so that is the default here. The US system, where weeks start on Sunday, is one click away.
Everything is worked out in your browser. Nothing is sent anywhere.
Features
- This week, big — the week number, its Monday to Sunday dates and a 7-day strip with today marked
- Date → week number — pick any date and get its week, its weekday number and the ISO code (like 2026-W46-3)
- Week number → dates — type a year and a week to get its first and last day
- Year table — all 52 or 53 weeks of any year from 1583 to 9999, with this week highlighted
- ISO or US weeks — Monday weeks (ISO 8601) or Sunday weeks (US), and the page remembers your choice
- Copy for Excel — copy the whole year table as plain text, ready to paste into a spreadsheet
- Your language — day and month names follow the language of the page
How ISO week numbers work
The ISO 8601 rule is short:
- A week starts on Monday and ends on Sunday
- Week 1 is the week with the first Thursday of the year (this is the same as the week with 4 January)
- A year has 52 or 53 weeks
Because of the Thursday rule, the first days of January can belong to the last week of the old year, and the last days of December can belong to week 1 of the new year. For example, 1 January 2027 is a Friday, so it is in week 53 of 2026. The week of 1 January 2026 had its Thursday in 2026, so 1 January 2026 is in week 1 of 2026.
| Year | Week 1 starts on | Weeks in the year |
|---|---|---|
| 2025 | Monday 30 December 2024 | 52 |
| 2026 | Monday 29 December 2025 | 53 |
| 2027 | Monday 4 January 2027 | 52 |
| 2028 | Monday 3 January 2028 | 52 |
| 2029 | Monday 1 January 2029 | 52 |
| 2030 | Monday 31 December 2029 | 52 |
Which years have 53 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. That happens about 5 or 6 times every 28 years.
Recent and coming years with 53 weeks: 2015, 2020, 2026, 2032, 2037, 2043 and 2048.
In a 53-week year, week 53 runs over New Year. In 2026, week 53 is Monday 28 December 2026 to Sunday 3 January 2027.
The US week system
In the US (and on many calendars in Canada and Japan) a week starts on Sunday, and week 1 is the week with 1 January. So the US number is often the same as the ISO number, often one higher, and in some years two higher. Around New Year the two can even count the same day in different years. A US year can also have 53 weeks.
This page keeps every week whole, from Sunday to Saturday. That means the last days of December can already count as week 1 of the next year, because that week holds 1 January.
Some programs cut that week in two instead. Excel's WEEKNUM(date) puts 31 December in week 53 (or 52) of the old year and starts week 1 on 1 January. Excel's ISOWEEKNUM(date) gives the ISO number, the same as this page in ISO mode.
How to use it
- Read this week's number at the top
- Choose ISO 8601 (Monday) or US (Sunday) if you need the other system
- To find the week of a date, pick it under Date → week number
- To find the dates of a week, type the year and week under Week number → dates
- Scroll to the year table, and use the arrows to see another year
- Press Copy the table to paste all weeks into Excel or Google Sheets
Frequently asked questions
What week is it today?
Look at the big number at the top of this page. It always shows the current week for the date on your device, with its Monday and Sunday.
What does "KW" mean?
KW is short for Kalenderwoche, German for "calendar week". KW 46 is week 46. German calendars use the ISO 8601 week, the default on this page.
Why is 1 January sometimes in week 52 or 53?
In the ISO system a week belongs to the year that has its Thursday. If 1 January is a Friday, Saturday or Sunday, its week has its Thursday in the old year. Then those first days of January are in the last week of the old year.
Do all countries use the same week number?
No. Most of Europe uses the ISO week, which starts on Monday. The US and some other countries start the week on Sunday and count from the week with 1 January. The numbers are often the same, but not always. Switch between the two at the top.
How can I get the week number in Excel or Google Sheets?
Use =ISOWEEKNUM(A1) for the ISO week. Use =WEEKNUM(A1) for the US week that starts on Sunday. Note that WEEKNUM cuts the week of New Year in two, so late December can differ from this page in US mode.
What is an ISO week date like 2026-W46-3?
It is year, week and day: 2026-W46-3 means the 3rd day (Wednesday) of week 46 of 2026. Day 1 is Monday and day 7 is Sunday.
Good to know
- Today's week comes from the date on your device
- The year table goes from 1583 (the first full year of the Gregorian calendar) to 9999
- The page moves on by itself at midnight if you leave it open
- Nothing you type is sent anywhere
Related tools
- Day of the Week Calculator — what day any date falls on
- Printable Calendar — print a month or a year, week numbers on or off
- 2027 Calendar — a ready calendar for next year
- Business Days Calculator — working days between two dates
- Date Calculator — add days or weeks to a date
- D-Day Calculator — count down to a day, D-100 to D-Day
