Week number

Which week is it

Pick a date and you get its ISO 8601 week number, together with the Monday and Sunday that bound it. The tool also works the other way round: type a week number and a year, and it prints the dates that week covers.

ISO 8601 is the numbering used across Europe in production plans, deliveries and rotas. Week one is the one that contains the first Thursday of the year, which is why the first days of January sometimes still belong to week 52 or 53 of the year before. The tool says so when it happens.

Around the date you also get the day of the year, the number of weeks in that year and the quarter. Everything runs in your browser, in UTC, so the result never shifts with a time zone.

Frequently asked questions

Why does 1 January sometimes fall in week 52?

Because an ISO week always runs Monday to Sunday and belongs to the year that holds most of it. If the year starts on a Friday, those first days still count towards the last week of the previous year.

Can a year have 53 weeks?

Yes. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. The tool shows the count for the year you are looking at.

Is this the same numbering the US uses?

No. In the US, weeks often start on Sunday and week one is the one containing 1 January. This tool uses ISO 8601 throughout, which is what business and manufacturing calendars in Europe follow.

What if I enter a week the year does not have?

It shows the last week of that year and says so, rather than silently rolling into the next January.