Week Number Calculator

Use this free week number calculator to find out what week of the year it is today, or the week number of any date, or the Monday to Sunday dates of any week number. The main answer is the ISO 8601 week used across Europe and in business software, and you also see the US week number. It shows how many weeks have passed and how many are left in the year. Tables show how many weeks are in each year and in a month, how many hours are in a week, and every week of the year with its dates. It is free and instant, and your date stays in your browser.

What week is it?

The calculator opens on today, taken from your own device, so the answer to “what week is it” is the first thing you see. The Today at a Glance box beside it also gives the day of the year and how many weeks have passed and are left. Pick another date, or use the Next week and Last week buttons, to see any other week.

How are weeks of the year numbered?

There is more than one way to number the weeks of a year, which is why two calendars can show different week numbers for the same day. This calculator shows the two most common ones. The main answer is the ISO 8601 week number, which is used across Europe, the UK and Australia and in most payroll, planning and business software. It also shows the US week number.

ISO 8601 weeks

• Weeks start on Monday and end on Sunday.
• Week 1 is the week that contains the first Thursday of the year. This is the same as the week that contains 4 January.
• A year has 52 or 53 weeks. A few days at the start or end of a year can belong to a week of the neighbouring year.

Which years have 53 weeks?

A year has 53 ISO weeks when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday. Recent and coming examples are 2015, 2020, 2026, 2032 and 2037. Every other year in that period has 52 weeks. You can check any year by entering it in the week-number-to-dates tab: asking for week 53 of a year that does not have one gives an error.

US week numbers

In the US, calendars and some software start the week on Sunday and call the week that contains 1 January week 1, even if only one day of that week is in the new year. That is why the US number can be one higher than the ISO number on the same date, and why it can reach week 53 at the end of a year.

How many weeks are in a year, a month or a quarter?

A normal year has 365 days, which is 52 weeks and 1 day, and a leap year has 52 weeks and 2 days. The ISO calendar numbers whole weeks, so it has 52 or 53 of them. A month is 4 weeks plus 0 to 3 days, which is about 4.35 weeks on average, and a quarter of a year is about 13 weeks. The Weeks in Each Year and Weeks in a Month tables on this page show the exact numbers.

Weeks into the year and weeks left

Weeks into the year counts the days from 1 January up to and including your date and shows them as weeks and days. Weeks left counts the days after your date to 31 December. The two always add up to the length of the year. On 28 October 2026 that is exactly 43 weeks passed and 9 weeks and 1 day left, which together make 52 weeks and 1 day.

Hours, minutes and seconds in a week

A week has 7 days, which is 168 hours, 10,080 minutes or 604,800 seconds. The How Long Is a Week table shows the days, hours and minutes in 1 to 52 weeks.

Week numbers in Excel and Google Sheets

In both programs, =ISOWEEKNUM(A1) gives the ISO week number of the date in cell A1. =WEEKNUM(A1) gives the US style number, where weeks start on Sunday and week 1 holds 1 January. =WEEKNUM(A1,21) gives the ISO number too.

Fiscal weeks

Some companies number weeks from the start of their fiscal year, which can be in any month. Those numbers will not match the calendar year numbers on this page, so check with your company which one it uses.

Finding the dates of a week

Use the second tab to go the other way: enter a year and an ISO week number, and the calculator shows the Monday and Sunday of that week. For example, week 40 of 2026 runs from Monday 28 September to Sunday 4 October. Week 1 of 2026 starts back on Monday 29 December 2025. The Week Numbers table at the bottom of the page lists every week of the year with its dates.

Calculators that go with this one

For the weekday of a date use the Day of the Week Calculator, to add weeks or days to a date use the Date Calculator, and to count the days between two dates use the Day Counter.