Week number calculator
See which week of the year it is today, or enter a date to find its week number, the Monday and the Sunday of that week, and how the US count of weeks differs.
ISO weeks start on Monday, and week 1 is the week that holds the first Thursday of the year, so a year has 52 weeks or 53. The US count starts the week on Sunday and makes week 1 the week that holds January 1. The days at the turn of the year can belong to the week-year before or after the calendar year.
How it works
The calculator numbers weeks the way ISO 8601 does. Weeks start on Monday, and the first week of a year is the one that holds its first Thursday. It finds the Thursday of the week of your date, and the week number is how many weeks that Thursday is into its year.
The rows below the number show the Monday and the Sunday of the week, the weeks in that year, and the day of the year. The page opens on today’s date.
The US count
The week data of Unicode for the United States starts the week on Sunday and makes week 1 the week that holds January 1. The two counts start their weeks on different days, so their numbers agree on some days and differ by one on others.
Sunday, November 1, 2026 is ISO week 44 and US week 45. Monday, November 2 is week 45 in both.
A worked example
Monday, December 29, 2025 starts ISO week 1 of 2026, because that week holds Thursday, January 1, 2026. 2026 has 53 weeks, since January 1 is a Thursday, so Friday, January 1, 2027 is still week 53 of 2026. In the US count the same Friday is week 1 of 2027.
Sunday, January 3, 2021 is week 53 of 2020 in the ISO count and week 2 of 2021 in the US one. Between 2015 and 2040 the years with 53 weeks are 2015, 2020, 2026, 2032 and 2037.
What it leaves out
The calculator counts weeks of the Gregorian calendar from the year 1583 to the year 9998. It does not know the week numbers of a school or a company, such as a fiscal week.
Sources
- WHATWG Living Standard, section 2.3.5.8, Weeks, read on 2026-11-02.
- Unicode CLDR, supplemental data: the first day of the week and the days in the first week, read on 2026-11-02.
Questions
What week of the year is it?
The page opens on today’s date and gives its week number in the first row. Type another date to find the week of that day.
How are ISO weeks numbered?
A week starts on Monday, and week 1 is the week that holds the first Thursday of the year. A year has 52 weeks or 53: 53 when January 1 is a Thursday, or a Wednesday in a leap year. 2026 has 53 weeks and 2025 has 52.
Can the last days of December be in week 1?
Yes. The days at the turn of the year can belong to the week-year before or after the calendar year. Monday, December 29, 2025 is week 1 of 2026, and Friday, January 1, 2027 is week 53 of 2026.
How does the US count of weeks differ?
In the data of Unicode for the United States a week starts on Sunday, and week 1 is the week that holds January 1. Sunday, November 1, 2026 is week 44 by the ISO count and week 45 by the US one.
How many weeks are in a year?
52 or 53 by the ISO count. A year of 365 days is 52 weeks and 1 day, and a leap year is 52 weeks and 2 days, which is why some years have a 53rd week.
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