What week is it? ISO and US numbers.

What week of the year is it depends on which rule counts the weeks, so the page above shows three side by side. Today falls in the ISO week, Monday to Sunday, and in the common US week, Sunday to Saturday. It also shows the CDC public health week. As an example, Monday, October 19, 2026 is in ISO week 43, US week 43 and CDC MMWR week 42. The week runs Mon, Oct 19, 2026 to Sun, Oct 25, 2026 in the ISO count and Sun, Oct 18, 2026 to Sat, Oct 24, 2026 in the US count.

How to use the page

1. Read the two big cards for the ISO week and the US week of the date in Date, which opens on today. Today puts it back.
2. Pick any other date in Date to see its week number and the dates of that week.
3. In The year by week, switch between ISO weeks, US weeks and CDC weeks to see the whole year as a grid. Today's week is purple. Click any week to open it above.
4. To go the other way, type a Year and a Week under Week number to dates, and choose the Week system.
5. Press Copy result for one sentence of text, or Copy link for a link that opens on the same date.

The three rules

The ISO rule is in the working draft of ISO 8601-1 that the Library of Congress hosts (PDF, a 2016 draft), and the ISO page on the standard is at iso.org. For the US common count, Microsoft's page for the spreadsheet function WEEKNUM describes the default the same way: week 1 contains 1 January and weeks start on Sunday (WEEKNUM). No official US standard for everyday week numbers is linked here, so treat the US column as a convention. The CDC rule comes from the MMWR week fact sheet. All of these pages were checked 19 Oct 2026.

Why some years have a week 53

Fifty-two weeks make 364 days, but a year has 365 or 366. The spare days build up, and every five or six years or so the ISO count needs a 53rd week. An ISO year has 53 weeks when 1 January is a Thursday, or when it is a Wednesday in a leap year. 2026 is one: 1 January 2026 is a Thursday. Its week 53 runs Monday, December 28, 2026 to Sunday, January 3, 2027. The page lists the 53-week years near the present; between 2010 and 2050 they are 2015, 2020, 2026, 2032, 2037, 2043, 2048.

Dates near New Year belong to the other year

A week can hold days of two years, so a date in late December can be in week 1 of the next year and one in early January can be in week 52 or 53 of the last. The table gives six dates and their weeks, so you can see where the three rules agree and where they do not.

The ISO week year can differ from the calendar year, which is why the page writes it as 2026-W53-5, year, week and weekday, the form the ISO standard uses. The US count here puts the whole week that holds 1 January in week 1, including its December days. Spreadsheet functions may split such a week in two, so check the function you use.

Which one should you use?

Use ISO when you work with Europe, with shipping and production schedules that name weeks, or with software that says ISO week. Use the US count when a US calendar or form tells you to. Use CDC weeks when you read US health reports. When a deadline says "week 43", ask which system it uses and, if you can, give the dates as well.

For the day number, use the day of the year calculator. For today in many formats, see today's date, and to count the days between two dates, use days between dates.