What week is it? Today’s week number

How week numbers work

Week numbers are widely used in Europe, Scandinavia and much of Asia for planning, payroll, shipping and school calendars ("delivery in week 43"). Most countries follow the international standard ISO 8601, which uses two simple rules:

1. Weeks start on Monday and end on Sunday.
2. Week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January.

As a result, a year has either 52 or 53 weeks. The first days of January can still belong to week 52 or 53 of the previous year, and the last days of December can already fall in week 1 of the next year. The year a week belongs to is called the ISO week-numbering year. Friday 1 January 2027, for example, is in week 53 of 2026.

Which years have 53 weeks?

A year has 53 ISO weeks if it starts on a Thursday, or if it is a leap year that starts on a Wednesday. 2026 starts on Thursday 1 January, so it has 53 weeks: week 53 runs from Monday 28 December 2026 to Sunday 3 January 2027. 2027 has 52 weeks. Recent 53-week years were 2015 and 2020, and the next one after 2026 is 2032.

How to calculate the week number yourself

week number = ⌊(day of year − day of week + 10) ÷ 7⌋

Here day of year is 1 for 1 January and 365 or 366 for 31 December, and day of week runs from 1 (Monday) to 7 (Sunday). If the result is 0, the date belongs to the last week of the previous year. If the result is 53 but the year only has 52 weeks, it is week 1 of the next year.

Example: Tuesday 6 October 2026 is day 279 of the year and day 2 of the week. (279 − 2 + 10) ÷ 7 = 41.0, so it is week 41.

US week numbers versus ISO week numbers

In the United States and Canada, calendars usually start the week on Sunday and count the week containing 1 January as week 1. That system can give a number one higher than the ISO week, especially early in the year. When you plan with international teams or suppliers, mention which system you use, or simply add the date.

Week numbers in Excel, Google Sheets and Outlook

• Excel: =ISOWEEKNUM(A1) returns the ISO week. The older =WEEKNUM(A1) starts weeks on Sunday by default; use =WEEKNUM(A1,21) for the ISO system.
• Google Sheets: =ISOWEEKNUM(A1).
• Outlook and Google Calendar: turn on week numbers in the settings and set the first day of the week to Monday. In Outlook, choose "First 4-day week" for ISO numbering.

Week numbers 2026

Week numbers 2027

Related tools

• Count the days until a deadline with the days between dates calculator.
• Add up the hours you worked this week with the hours calculator.
• Pregnancy weeks are counted differently from calendar weeks; see the due date calculator.

Last reviewed: 2026-10-06. Results are estimates for information only.