Week Number Calculator

Shows which week of the year it is today and converts a date to its week number and back. Use it for weekly reports, schedules or a week of the month such as "October, week 2".

How to use

1. Today's week, the weeks in the year and the weeks left appear at the top.
2. Under "Week numbering", choose "ISO 8601", "US" or "Week of the month"; for the last one, also choose a "Week-of-month rule".
3. Enter a date under "Date to week", or a year and week number under "Week to dates" to get that week's first and last day.
4. Under "Weeks of the year", see every week of a year and press "Print". Matching spreadsheet formulas can be copied.

How it works

• ISO 8601 weeks start on Monday, and week 1 is the week with the year's first Thursday.
• US weeks start on Sunday, and week 1 is the week with January 1, so a date can have a different number than in ISO. This page starts with US weeks.
• For the week of the month, choose "First Thursday" or "Week with the 1st is week 1". Years from 1601 to 9999 can be entered.

Questions and answers

What week number is it today?

It appears in large type at the top of the page. Companies and countries count weeks differently, so check the "Week numbering" setting.

How many weeks are in a year?

An ISO year has 52 or 53 weeks. It has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.

Why is January 1 in the last week of the previous year?

In ISO 8601, week 1 contains the year's first Thursday. A January 1 that falls on a Friday, Saturday or Sunday belongs to the last week of the year before.

Which Excel function gives the week number?

=ISOWEEKNUM(A1) for ISO weeks and =WEEKNUM(A1,1) for US weeks. The page shows the formulas for the system you chose.