Pick a date to see its week number and the dates the week runs from and to. You can also look up a week number or list every week of a year.
ISO week
2026-W01
Week 1 of 53 in 2026
Week runs Mon–Sun
Week of the month
The week that contains the 1st of the month counts as week 1.
Weeks in January
What an ISO week number is and how it is counted
The ISO 8601 standard defines weeks that start on Monday, and it counts as week 1 the week that contains the first Thursday of the year. Pick a date and the tool shows its ISO week number and the Monday and Sunday that bound the week.
The same page works in the other direction and in bulk. You can enter a week number to get its dates, open a calendar of every week in a year, and see which week of the month a date falls in. Switch to the US system when you need Sunday-start weeks that match Excel's default WEEKNUM.
Everything is calculated in your browser.
When a week number is the date
A deadline written as Week 22
A supplier or a project plan gives only a week number. Enter the year and the week to see the exact Monday and Sunday.
Sprint and production planning
Open the Year calendar, then copy the week list into a sheet to use as the base of a plan.
Week numbers that do not match
Two reports show different weeks for the same date. Switch between ISO and US numbering to see which one each report uses.
Dates around New Year
Late December can be week 1 of next year and early January can be the last week of the old one. The tool flags these dates.
Which week of the month is it
See the week of the month for both Sunday-start and Monday-start calendars, and whether it is the last week.
How to find a week number in 4 steps
- Choose the numbering system. ISO 8601 with Monday start is the default. Pick US for Sunday-start weeks.
- On the Date to week tab, choose a date. The page opens on today. You get the week code, such as 2026-W44, the start and end dates, and the week of the month.
- For a deadline given as a week number, open Week to dates and enter the year and the week.
- Open Year calendar to see every week of a year, and use Copy week list to paste it into a spreadsheet.
Note: Click any week in a list to open it on the Date to week tab. The Today button returns to the current date.
Rules behind the numbers
- ISO 8601: weeks run Monday to Sunday and week 1 contains the year's first Thursday. So January 1 can be in the last week of the previous year, and December 29 to 31 can be in week 1 of the next.
- US numbering: weeks run Sunday to Saturday, and the week that contains January 1 is week 1. Weeks are cut at the year boundary, so the first and last weeks can be shorter than seven days.
- Week codes: a week is written as year, W, and the week number, such as 2026-W44. In ISO mode the tool also shows the full ISO week date, which adds the weekday number from 1 for Monday to 7 for Sunday, as in 2026-W44-7.
- 53 weeks: an ISO year has 53 weeks when January 1 is a Thursday, or a leap year that starts on a Wednesday. A US year can have 53 or 54.
- Week of the month: the week with the 1st is week 1, counted for both Sunday and Monday starts.
- Years: the year fields accept 1000 to 9999.
Dates near New Year and their ISO weeks
These are the dates where the calendar year and the ISO week-year differ or where the week count is easy to get wrong.
| Date | ISO week | Week runs (Mon–Sun) | Why |
|---|---|---|---|
| Dec 30, 2024 | 2025-W01 | Dec 30, 2024 – Jan 5, 2025 | Calendar year 2024, ISO year 2025 |
| Dec 28, 2025 | 2025-W52 | Dec 22 – Dec 28, 2025 | Last week of 2025 |
| Dec 29, 2025 | 2026-W01 | Dec 29, 2025 – Jan 4, 2026 | Still December, already week 1 of 2026 |
| Jan 1, 2026 | 2026-W01 | Dec 29, 2025 – Jan 4, 2026 | The week holds Thursday, Jan 1 |
| Dec 31, 2026 | 2026-W53 | Dec 28, 2026 – Jan 3, 2027 | 2026 has 53 weeks |
| Jan 3, 2027 | 2026-W53 | Dec 28, 2026 – Jan 3, 2027 | Calendar year 2027, ISO year 2026 |
| Jan 4, 2027 | 2027-W01 | Jan 4 – Jan 10, 2027 | First week of 2027 |
| Jan 2, 2028 | 2027-W52 | Dec 27, 2027 – Jan 2, 2028 | Calendar year 2028, ISO year 2027 |
Why the numbers differ between systems. The ISO rule gives each week to the year that holds its Thursday, so no week is split. The US rule counts the week that has January 1, and cuts it at the year boundary. For the same date, the two numbers can differ by one.
Years with 53 ISO weeks. Examples are 2020, 2026, 2032, and 2037. If a system assumes 52 weeks, its year-end process can fail in those years.
Week numbers in spreadsheets, databases, and code
Where ISO and US week numbers diverge in everyday tools, and how to keep them consistent.
Matching Excel and Google Sheets
WEEKNUM(date) counts US weeks by default, with Sunday as the first day. For ISO weeks use ISOWEEKNUM(date), or WEEKNUM(date, 21). Switch this tool to US numbering to see the value the default WEEKNUM returns, and to ISO to see the other.
Grouping weekly data across New Year
If you group by calendar year and week number, the week of Dec 29, 2025 to Jan 4, 2026 splits into two groups. Group by ISO week-year and week instead. In Python, date.isocalendar() returns both. In PostgreSQL, use EXTRACT(ISOYEAR FROM date) with EXTRACT(WEEK FROM date).
Planning with 52, 53, or 54 weeks
A year is 52 weeks and one or two days, so some years need a 53rd week. The Year calendar tab shows the count for the year and the numbering you pick, and warns when it is 53 or 54. Check it before you fix a limit in a table or a report.
Time zones when you parse dates in JavaScript
new Date("2026-01-01") is read as midnight UTC, which can show as the previous day in time zones west of UTC. Around New Year, one day can change the week number. This tool builds each date from its year, month, and day in local time, so the result does not depend on the time zone.
Why week numbers come out wrong
WEEKNUM does not match the ISO week
Typing WEEKNUM(date) in Excel or Google Sheets returns the US week, so a report shared with another office can be off by one week without anyone noticing.
💡 See the solution▼
Different starts for week of the month
August 1, 2026 is a Saturday. With a Sunday start, August 2 begins week 2. With a Monday start, August 2 is still in week 1. A plan that says week 2 can mean two different weeks.
💡 See the solution▼
Splitting a week by calendar year
Using the calendar year with the week number puts December 29 to 31 and January 1 to 4 in different groups, which makes weekly totals drop at New Year.
💡 See the solution▼
Assuming a year always has 52 weeks
A table or script that stops at week 52 fails in the years that have a week 53, such as 2026 and 2032.
💡 See the solution▼
Reading a date in the wrong time zone
A date string like 2026-01-01 can be read as UTC midnight and shift by a day in some time zones, which changes the week number around New Year.
💡 See the solution▼
Week number FAQ
Q.What is an ISO week number?
A. It is the week count defined by the ISO 8601 standard. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. Because of that rule, 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 year.
Q.What week number is it today?
A. Open the page and the Date to week tab already shows today's date, with the week number and the Monday-to-Sunday dates. If you pick another date, click Today to return. Use the numbering buttons to switch between the ISO week and the US week.
Q.How do I find the dates of a week number, such as week 22?
A. Open the Week to dates tab and enter the year and the week number. In ISO numbering for 2026, week 22 runs from May 25 to May 31, week 36 from August 31 to September 6, week 46 from November 9 to November 15, and week 50 from December 7 to December 13.
Q.How many weeks are in a year?
A. A year has 52 weeks plus one or two extra days, so an ISO year has either 52 or 53 weeks. In the US system a year can have 53 weeks, or 54 in years such as 2028. The Year calendar tab shows the total for the year and the numbering you choose.
Q.Which years have 53 ISO weeks?
A. An ISO year has 53 weeks when January 1 falls on a Thursday, or when it is a leap year and January 1 falls on a Wednesday. Recent and upcoming examples are 2015, 2020, 2026, 2032, and 2037.
Q.Why does December 29 show as week 1 of the next year?
A. ISO weeks belong to the year that holds their Thursday. If a week runs from Monday, December 29 to Sunday, January 4, its Thursday is January 1, so the whole week counts as week 1 of the new year. The tool shows a notice when the calendar year and the ISO week-year differ.
Q.How is the US week number different?
A. US weeks start on Sunday, and the week that contains January 1 is week 1. The week is cut at the end of the year, so in 2026 week 1 is only January 1 to January 3. This is the default behavior of the WEEKNUM function in Excel.
Q.How do I get ISO week numbers in Excel or Google Sheets?
A. Use ISOWEEKNUM(date). WEEKNUM(date) counts US weeks by default, but WEEKNUM(date, 21) returns the ISO week. You can check any result against this tool by switching between the ISO and US numbering.
Q.What does week of the month mean here?
A. The week that contains the 1st of the month counts as week 1. The tool shows the result for both Sunday-start and Monday-start calendars, because the numbering differs. For September 2026, the last week is September 27 to 30 with a Sunday start, and September 28 to 30 with a Monday start.
Q.Can I copy a whole year of weeks into a spreadsheet?
A. Yes. On the Year calendar tab, click Copy week list. It copies every week with its start and end date, separated by tabs, with dates as YYYY-MM-DD. Paste it into Excel or Google Sheets and each value lands in its own column.
Q.Are my dates sent to a server?
A. No. The week numbers are worked out in your browser with JavaScript, and nothing is sent to a server. The tool does not save the dates you pick.
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