Current week
40
Today, Thursday, October 1, 2026, is in week 40 of 2026.
Week 40 runs from Monday, September 28, 2026 to Sunday, October 4, 2026.
ISO 8601 week numbering (Monday to Sunday), the standard used in business planners and Excel's ISOWEEKNUM.
In ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday (so it always contains January 4). A year has 52 or 53 weeks, and the first days of January can belong to the last week of the previous year. The common US system starts weeks on Sunday and makes the week containing January 1 week 1, so its number can be one higher.
2026 has 53 ISO weeks. Week 1 starts on Monday, December 29, 2025 and the last week ends on Sunday, January 3, 2027.
| Week | Starts (Monday) | Ends (Sunday) |
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| Week 40 | ||
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| Week 52 | ||
| Week 53 |
Today's week number is shown at the top, with the Monday it starts on and the Sunday it ends on.
In "Which week is a date in?", pick any day to see its ISO week number and the US-style week number.
In "Which dates are in a week?", type a week number and a year to see the dates it starts and ends.
Further down you will find every week of the year with its dates, with the current week highlighted.
The international standard ISO 8601 says weeks start on Monday and that week 1 is the week containing the first Thursday of the year. In practice, week 1 is always the week that contains January 4, and every week belongs entirely to one numbering year.
Because of this rule a year has 52 or 53 weeks. January 1–3 can belong to week 52 or 53 of the previous year, and the last days of December can already be in week 1 of the next year. This is the numbering used in business planners, manufacturing, logistics and most of Europe.
The system common in the United States starts weeks on Sunday and calls the week containing January 1 week 1, even if it has only one or two days. Its number is often one higher than the ISO week. The date lookup shows both numbers so you can match whichever system your team uses.
In Excel and Google Sheets, ISOWEEKNUM returns the ISO number shown on this page; WEEKNUM uses the US system by default.
The current week number is shown at the top of this page, calculated from your device's date with the ISO 8601 standard, along with the week's start and end dates.
Under ISO 8601, 52 or 53. A year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year. 2026 is one of them.
ISO weeks, used on this page, start on Monday. US calendars usually start on Sunday, which can give a different week number.
Use =ISOWEEKNUM(A1) for the ISO week, the same as this page. =WEEKNUM(A1) counts weeks starting on Sunday and may give a different number.
Because ISO week 1 is the week containing the year's first Thursday. When January 1 falls on a Friday, Saturday or Sunday, those days still belong to the last week of the previous year.
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