Week Number

How to read a week number

1. 1 The date field starts on today, so the big figure is the week you are in right now. Change the date to look up any other day.
2. 2 The dates underneath are the Monday and the Sunday of that week. ISO weeks always start on Monday, whatever your calendar app shows.
3. 3 The second section works backwards: type a week number and a year to see which dates it covers. This is what you need when a supplier promises delivery in week 42.
4. 4 Copy the address bar to share a lookup; the date you picked travels with the link.

Frequently asked questions

How is week 1 decided?

ISO 8601 puts week 1 on the week holding the first Thursday of January, which is the same as saying the week holding 4 January. So the first days of January often belong to the last week of the previous year, and the tool shows that year rather than pretending otherwise.

Why does my calendar show a different number?

Some calendars, mostly in North America, start weeks on Sunday and call week 1 the one containing 1 January. That system can differ from ISO by one. Business, logistics and manufacturing outside the US run on ISO, which is what this page uses.

Can a year have 53 weeks?

Yes. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year. Most years have 52. The line under the result tells you which kind of year you are looking at, because planning spreadsheets built for 52 weeks quietly break in the other kind.

What does KW mean?

Kalenderwoche, the German for calendar week. German-speaking business runs on week numbers to an extent that surprises visitors, so a delivery date given as KW 42 is completely normal there. It is the same ISO number shown here.