Week number: what week is it?

What week is it?

The big figure at the top, under "Current week", is today's week number. Below it you get the Monday and Sunday that frame the week, today's date written out, and the day of the year. On Friday, November 13, 2026, the page shows 46, "Monday, November 9 to Sunday, November 15, 2026" and "Day 317 of the year, 48 days left".

The number follows ISO 8601, the international standard, and is worked out in your browser from your device's date.

How to use it

1. "Week number of a date": pick any date to get its week number, the year that week belongs to, and its Monday to Sunday dates. "Today" jumps back to the current date.
2. "Dates of a week": enter a "Year" and a "Week number". Week 27 of 2026 runs from Monday, June 29 to Sunday, July 5.
3. "Week calendar": all week numbers for 2026, or any other year, each with its Monday and Sunday. "Previous year" and "Next year" move around; "Show current week" returns to the highlighted row for this week.
4. "Copy" gives a ready-made sentence such as "Week 46: Monday, November 9 to Sunday, November 15, 2026". "Copy link" gives an address ending in #2026-11-13 or #2026-W46 that reopens the page on the same date or week.

The ISO rule in plain words

Weeks run from Monday to Sunday. Week 1 is the week that contains the first Thursday of the year: the first week with at least four days in the new year. January 4 always falls in it.

Because of that, the first and last days of a year can belong to a week of the neighbouring year:

• Friday, January 1, 2021 was in week 53 of 2020;
• Monday, December 30, 2024 was already in week 1 of 2025;
• week 1 of 2026 ran from Monday, December 29, 2025 to Sunday, January 4, 2026.

This is why the tool always shows the year of the week.

Why some years have 53 weeks

A year is 52 weeks plus one day, or two days in a leap year. Those leftover days eventually add up to an extra week. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.

2026 starts on a Thursday, so it has 53 weeks. The next ones are 2032, 2037, 2043 and 2048; the last one was 2020. Ask for week 53 of 2025 under "Dates of a week" and the tool refuses it, because 2025 has only 52.

ISO weeks and North American weeks

In the United States and Canada, calendars usually start the week on Sunday, and week 1 is simply the week that contains January 1, even if only one of its days is in the new year.

This page shows both: the line "North American numbering (not ISO)" sits under the current week and under any date you pick. Sunday, January 4, 2026 is ISO week 1 but North American week 2. Friday, January 1, 2021 is ISO week 53 of 2020 but North American week 1 of 2021.

Where week numbers are used

In the UK, Ireland and the rest of Europe, week numbers turn up in project plans, factory schedules, delivery slots ("shipping in week 47"), shift rotas and weekly payroll. American companies with European suppliers meet "wk 46" on order confirmations and need to know it means the ISO week.

Showing week numbers in a calendar or spreadsheet

Most calendar apps, on phones and computers, have a setting to show week numbers; once it is on, the number appears beside each row of the month view. In a spreadsheet, look for the dedicated ISO week number function. The plain week number function usually defaults to the Sunday-based count, so it can be one off.

Frequently asked questions

Does the week start on Monday or Sunday?

On Monday under ISO 8601. The North American line counts from Sunday.

Is January 1 always in week 1?

Not under ISO. If it falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year. January 1, 2027 is in week 53 of 2026.

How many weeks are there in 2026?

53. The line above the calendar table gives the count for each year: 52 for 2025, 53 for 2026.

What is the last week of 2026?

Week 53, from Monday, December 28, 2026 to Sunday, January 3, 2027.