How to Find the Current Week Number

The fastest way

Open the first tab of the calculator on this page. The date field starts on today's date, so the large number is the current ISO week number. The sentence below it names the Monday and Sunday of this week, and the rows underneath show the week-year, the weekday and the day of the year. Nothing to type, nothing to sign up for.

As an example, on Thursday 22 October 2026 the page shows 43, and the week runs from Monday 19 October to Sunday 25 October. On Monday 26 October the number changes to 44. A new ISO week always begins on a Monday at midnight, never in the middle of a week.

Show week numbers in your calendar app

Most calendar programs can display a week number next to each row of the month view. Look in the settings for an option called "show week numbers" or "week numbers". It exists in Google Calendar, in the Apple Calendar app on iPhone, iPad and Mac, and in Outlook.

Be careful with one detail. Some apps follow the rules of your region, and some let you choose the first day of the week and which week counts as week 1. In a setting where the week starts on Sunday, the numbers can differ from the ISO numbers around New Year. If you want ISO numbers, set the week to start on Monday and use the rule that week 1 must have at least four days in the new year.

A spreadsheet formula

In Excel and in Google Sheets the function ISOWEEKNUM returns the ISO week number of a date. The formula =ISOWEEKNUM(TODAY()) gives the current week and updates every day. The older function WEEKNUM works differently: by default it starts weeks on Sunday and counts the week with 1 January as week 1, which is the US rule.

If you want the week-year as well, remember that it can differ from the calendar year. Around New Year, ISOWEEKNUM returns 52 or 53 for the first days of January and 1 for the last days of December. A report that sorts by calendar year and week number must use the ISO week-year, not the year of the date, or the rows will be in the wrong order.

Programming and the command line

Most programming languages can give the ISO week. In Python, date.isocalendar() returns the week-year, the week number and the weekday. In PostgreSQL, EXTRACT(WEEK FROM date) follows the ISO rule. In MySQL, WEEK(date, 3) gives the ISO number, while the default mode does not. On Linux and macOS, the command date +%V prints the ISO week number.

JavaScript has no built-in function for it, which is why many websites show wrong numbers. A correct version finds the Thursday of the week of the date and counts the weeks from 1 January of that Thursday's year. That is exactly the method behind this calculator.

A mental shortcut

You can get close in your head. Divide the day of the year by 7 and round up. The 295th day of the year is 22 October in 2026. 295 divided by 7 is 42.1, which rounds up to 43, the correct ISO week. The shortcut is exact for many dates but can be off by one when the year does not start on a Monday, so use it as a sanity check and not as the answer.

For a more careful check, use the weekday of 1 January. If 1 January is Monday, Tuesday, Wednesday or Thursday, it is in week 1. If it is Friday, Saturday or Sunday, it is in the last week of the previous year, and week 1 starts on the following Monday. Count weeks from there. In 2026, 1 January is a Thursday, so week 1 starts on Monday 29 December 2025.

Why two sources can show different numbers

If your phone says week 43 and a colleague says week 44, they are probably using different rules. The ISO rule starts the week on Monday and needs four days in the new year. The US rule starts the week on Sunday and makes the week with 1 January week 1. In the middle of the year the two usually agree, because most weeks do not touch New Year. Near the turn of the year they can differ by one or more.

Another cause is a time zone. Late on Sunday evening in one country it is already Monday in another, so a date can fall into the next week there. The calculator avoids that problem by asking for a date, not a time. A given calendar date has one ISO week number everywhere.

When a deadline depends on the number, write the date range next to it, for example "week 47, 16 to 22 November 2026". It costs a few words and removes the doubt.

The first day of each month in 2026

A table of month starts helps you place a date quickly. In 2026, 1 January is in week 1, 1 February in week 5, 1 March in week 9, 1 April in week 14, 1 May in week 18 and 1 June in week 23. After that, 1 July is in week 27, 1 August in week 31, 1 September in week 36, 1 October in week 40, 1 November in week 44 and 1 December in week 49.

You can use these anchors to work out any date in the month. For 15 September 2026, start at week 36, which contains 1 September, and add two weeks, because 15 September is 14 days after it. The answer is week 38. Always check with the calculator when the date is within a few days of a month start, because the Monday of a week can lie in the previous month.

What changes when you travel

The ISO week number does not depend on where you are. Week 43 of 2026 is week 43 in Berlin, Tokyo and Chicago, because a calendar date has the same week everywhere. What does change from country to country is the habit of using week numbers at all, and the rule people apply when they do.

If you work with a team abroad, state the rule once. A phrase such as "ISO weeks, Monday to Sunday" in a project plan is enough. Without it, a partner in the United States who counts weeks from Sunday may read week 44 where you mean week 43.