Week number: what week is it today (ISO and US)

How ISO week numbers work

ISO 8601 weeks start on Monday and end on Sunday. Week 1 of a year is the week that contains that year’s first Thursday. Two other rules say the same thing: week 1 contains 4 January, and it is the first week with at least four days in the new year.

Each day is written as an ISO week date: the week-numbering year, W and the week number, then the weekday from 1 (Monday) to 7 (Sunday). Friday 23 October 2026 is 2026-W43-5, or 2026W435 in basic form.

52 or 53 weeks, and years that start early

A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. All other years have 52. 2026 starts on a Thursday, so it has 53 weeks. 2027 has 52.

Because weeks are never split, the ISO week-numbering year can differ from the calendar year for a few days around 1 January:

28 December is always in the last week of its year, and 4 January is always in week 1. When you label data by week, store the ISO year with the week, or 1 January 2027 will sort into the wrong year.

US week numbers are a different system

Many US calendars and tools number weeks differently: weeks start on Sunday, and week 1 is the week that contains 1 January, however few days of it fall in the new year. The converter shows this number separately and labels it as the US system.

The two systems differ on Sundays, which close an ISO week but open a US one, and around New Year. A US year always starts at week 1 on 1 January, so its last week can be a partial week of only a few days. When a colleague or client quotes a week number, confirm which system they use.

Week numbers in Excel and code

Excel has two functions, documented by Microsoft:

• =ISOWEEKNUM(A1) returns the ISO week. It is available in Excel 2016 and later, including Microsoft 365.
• =WEEKNUM(A1) uses return type 1: weeks start on Sunday and the week containing 1 January is week 1. That is the US system.
• =WEEKNUM(A1,21) uses Microsoft’s System 2, which the documentation describes as the ISO 8601 method.

Neither function returns the ISO year, so for dates in early January or late December add it yourself. To get the Monday of an ISO week from a date in A1, use =A1-WEEKDAY(A1,2)+1.

In code:

• Python: date(2027, 1, 1).isocalendar() returns year 2026, week 53, weekday 5, and date.fromisocalendar(2026, 43, 1) returns Monday 19 October 2026.
• PostgreSQL: EXTRACT(WEEK FROM d) is the ISO week and EXTRACT(ISOYEAR FROM d) is its year.
• MySQL: WEEK(d, 3) returns ISO weeks; the default mode does not.

Where week numbers are used

Week numbers give a short label for a seven-day period, so they turn up in project schedules, delivery and production plans, and weekly reports. ISO weeks are the safer choice when people in different countries share a plan, because the rules are fixed by the standard. The ISO 8601 week date also appears in data files, alongside the calendar and ordinal forms covered on the ISO 8601 page.

For a full list of weeks with their Monday and Sunday dates, see the 2026 week number calendar and the 2027 week number calendar. For the position of a day within the year, use the day of the year converter.

Questions