In short: The week number of a date is its position in the year's Monday to Sunday weeks under ISO 8601, where week 1 is the week with the first Thursday of January. 2026 has 53 ISO weeks and 2027 has 52. The calculator shows today's week, the week of any date and the dates of any week.
How week numbers work
Week numbers come from ISO 8601, the international standard for writing dates and times. It defines a calendar week like this:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week with the first Thursday of January. Put another way, it is the week that contains 4 January, because a week starting on Monday always holds a Thursday within its first four days.
- Weeks are numbered 1 to 52 or 53 from there, and the last week of the year is the one with the last Thursday of December.
- The week-year follows the week, not the calendar. If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year. If 31 December falls on a Monday, Tuesday or Wednesday, it belongs to week 1 of the next year.
So Friday 1 January 2027 is in week 53 of 2026, and Monday 30 December 2024 is in week 1 of 2025. A date written as a week date, such as 2026-W41-6, means the sixth day (Saturday) of week 41 in the 2026 week-year, which is 10 October 2026.
ISO week numbers and US week numbers
Many calendars in the United States, Canada and some other countries count weeks differently: the week starts on Sunday, and week 1 is whichever week contains 1 January, even when that week has only one day in the new year. This way of counting is not standardised, which is why this page labels it "US (Sunday start)". Under it, 1 January is always in week 1, and the end of December always reaches week 53, or 54 in a leap year that starts on a Saturday, because week 1 is seldom a full week.
The two systems give the same number for much of the year but drift apart on Sundays, where the US week has already moved on while the ISO week has not, and for a few days around 1 January. Spreadsheets expose both: Excel's WEEKNUM function uses the Sunday-start rule by default (its "System 1") and switches to the ISO rule with a return type of 21, while ISOWEEKNUM always gives the ISO week. Outlook and Google Calendar have a setting for the first day of the week and for which week counts as week 1, so two colleagues can see different numbers for the same day until their settings match.
Years with 53 weeks
Fifty-two weeks make 364 days, one or two short of a calendar year. The spare days pile up, and roughly every five or six years they fill a whole extra week. A year has 53 ISO weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year. 2026 is one of them: it starts on a Thursday, so week 53 runs from 28 December to 3 January 2027. 2027 has 52 weeks, and the next 53-week year after 2026 is 2032.
A 53-week year matters for anything planned by week: a weekly payroll pays 53 times, a weekly report has one more issue, and a budget split over 52 weeks ends a week early. Check the year page before setting up a weekly schedule.
Week numbers 2026 and 2027
The week numbers pages list every ISO week of 2026 and 2027 with its Monday and Sunday dates and the months it spans, and mark the current week. For deadlines counted in working days rather than weeks, use the business days calculator.
Questions people ask
What week is it now?
The line at the top of this page shows the current ISO week number, worked out by your browser from your device's date, with the Monday and Sunday of the week and the US (Sunday start) number beneath it. Nothing is sent to a server. If the number looks wrong, check the date on your device.
Why does my calendar show a different week number?
Calendars count weeks in two main ways. The ISO 8601 rule starts weeks on Monday and makes week 1 the week with the first Thursday of January. The common US rule starts weeks on Sunday and makes week 1 the week that contains 1 January. Near the start of the year the two can differ by one, and on Sundays they differ for most of the year. Outlook, Google Calendar and Excel each let you choose the rule in their settings.
How many weeks are in 2026 and 2027?
2026 has 53 ISO weeks because it starts on a Thursday. 2027 has 52. A year has 53 ISO weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year; every other year has 52.
When does week 1 start?
ISO week 1 starts on the Monday of the week that contains 4 January, which is the same as the week with the first Thursday of the year. For 2026 that Monday is 29 December 2025; for 2027 it is 4 January 2027. Under the US rule week 1 starts on the Sunday on or before 1 January.
Is week 53 real?
Yes, in some years. 52 weeks are only 364 days, so a year of 365 or 366 days leaves one or two days over, and every five or six years they add up to a full extra week. 2020, 2026 and 2032 each have 53 ISO weeks. In the other years the days left over belong to week 1 of the next year or week 52 of the previous one.
Does the ISO week-year differ from the calendar year?
It can, for a few days at each end of the year. 1 January 2027 is a Friday, so it belongs to week 53 of 2026, and 30 December 2024 belongs to week 1 of 2025. The calculator always shows the week-year with the week number so the pair is never ambiguous.
Is my date sent anywhere?
No. The week is worked out in your browser. A share link carries the date or week in the part after the # in the address, which browsers never send to a server.