ISO week numbers

There are several week numbering schemes. The most widespread is the one used on this site. It is commonly known as ISO week numbers, named after the ISO 8601 computing standard for exchange of date and time-related data.

Week numbers are not used much in the United States, but the country has its own numbering scheme, just like it uses different units for weights and measures than most other countries. Unfortunately, the US week numbering scheme is often the default scheme used in software, because a lot of software originates there. Weeknumber.ie exclusively deals with ISO week numbers

First day of the week

According to ISO 8601, the first day of the week is Monday and the last day of the week is Sunday. Some countries have different conventions, but in any case the ISO week number changes at midnight between Sunday and Monday.

For week numbering purposes, a week always contains 7 days, i.e. even if New Year falls within a week, the week number stays the same until next Monday. This means that week 1 sometimes begins on a Monday in the last few days of December, and sometimes week 52 ends on a Sunday in the first few days in January. Only when New Year’s Eve is a Sunday will the week number change exactly at New Year.

How to determine week 1

There are different ways to determine which week 1 in a given year:

• The week containing 4 January
• The week containing the first Thursday in January
• The first week of the year containing at least four days within January

These might sound like conflicting definitions, but if you look up a few different years in the calendar, you will realise that they are indeed equaivalent.

Once you have found week 1, the next seven days are obviously week 2 and so on, so based on that you can determine the week numbers for any given day in a year.

Number of weeks in a year

Most years have 52 weeks. But 52 × 7 is only 364, and there are 365 days in a year – or even 366 on a leap year – so every few years you need a “leap week”, i.e. week 53, to make the numbers add up.

According to the definition above, week 1 always contains 4 January, so seven days earlier, 28 December, must contains the last week of the previous year. So to get the number of weeks in a year, either 52 or 53, you can always look up the week number of 28 December.

According to a different (but equivalent) definition, a year has 53 weeks, if either 1 January or 31 December is a Thursday.