How Many Weeks in a Year?

Every year has 52 weeks. The year 2023 also has 52 weeks. At least mathematically, because while in purely mathematical terms every year consists of 52 weeks and 1 or 2 days, there are also 53 and 54 weeks per year in the case of numbered calendar weeks. We'd like to demonstrate how this works in the following section:

How many weeks does a year have, mathematically?

A year has about 52 weeks.

A normal (non-leap) year has 365 days. A week has 7 days. Thus, a year consists of 52 weeks and one day, because 52 times 7 is 364 days, and the one extra day is 365 days, i.e., one year. Or, to put it another way, 365 days divided by 7 equals 52.14, or 52 weeks and one day. A leap year also has 52 weeks. A leap year occurs every four years and has 366 days. That’s 52 times 7 days, so 52 weeks and another 2 days.

This is the purely mathematical method of determining the weeks of a year by dividing the days of a year by the 7 days of the week. However, there are also definitions according to which the weeks of a year are numbered as calendar weeks from the first to the last week. This is widely used, especially in business. With the different methods of numbering, there can also be 53 or even 54 weeks per year.

How many weeks, according to ISO, does a year have?

Each year has either 52 or 53 calendar weeks, according to the ISO standard.. This depends on which day of the week the year begins.

The international ISO definition of a week is predominantly used in European and Asian countries to number weeks. The definition states that a week always starts on a Monday, and the first week of the year always includes the first Thursday of the year.

When does a year have 52 weeks according to ISO?

Normally, a year has 52 ISO calendar weeks, as the following example illustrates. There, January 1 is a Tuesday, so only one day of the first week is in the old year. Thus this is the first calendar week. This is followed by 51 weeks, i.e., 51 times 7 equals 357 days. The week that follows still begins in the current year, but has more days in the following year. Therefore, this week already forms the first calendar week of the following year. This example year therefore has 52 calendar weeks.

Example for 52 ISO weeks at 365 days
	Mo	Tu	We	Th	Fr	Sa	Su
CW 1		01/01	02/01	03/01	04/01	05/01	06/01
CW 2 to CW 52	51 times 7 equals 357
CW 1 of the New Year	29/12	31/12

When does a year have 53 weeks, according to the ISO standard?

If a year (not a leap year) begins with a Thursday, it has 53 calendar weeks. First, three days of the first calendar week (Monday 29/12, Tuesday 30/12 and Wednesday 31/12) are carried over from the previous year. Likewise, three days of the last calendar week are in the next year. This is because a 365-day year that begins on a Thursday also ends on a Thursday. Thus, Friday, January 1, to Sunday, January 3, are also in the last calendar week. This total of 6 days plus the 365 days gives 371 days. Divided by 7, this gives exactly 53 calendar weeks.

Example for 53 ISO weeks at 365 days
	Mo	Tu	We	Th	Fr	Sa	Su
CW 1				01/01	02/01	03/01	04/01
CW 2 to CW 52	51 times 7 equals 357
CW 53	28/12	29/12	30/12	31/12

A leap year with 366 days has 53 calendar weeks if it begins on a Wednesday and ends on a Thursday, or if it begins on a Thursday and ends on a Friday.

How many North American weeks are there in a year?

Each year has either 53 or 54 North American calendar weeks. This again depends on which day of the week the year begins.

In the USA, Canada, almost all of South America, Australia, as well as Japan, Israel, South Korea, etc., a different system for numbering weeks is used compared to the ISO standard. Here, Sunday is the first day of the week, and Saturday is the last day of the week. In addition, the first calendar week always begins on January 1, regardless of the day of the week. Thus, the first and the last week of a year are usually not 7 days long. This is because the first calendar week starts – regardless of the day of the week – on January 1 and ends on the following Saturday, and the last calendar week always starts on the last Sunday of the year and ends on December 31.

Is there also a year with 52 weeks according to the North American definition?

No. Even if, for example, the first week of the year begins on a Sunday (1 January), and consists of a full 7 days, a year has at least 365 divided by 7 = 52 weeks and one day. This one day is then the Sunday on 12/31 of that year and counts as the 53rd calendar week, as shown in the table below.

Example for 53 North American Calendar weeks at 365 days
	Su	Mo	Tu	We	Th	Fr	Sa
CW 1	01/01	02/01	03/01	04/01	05/01	06/01	07/01
CW 2 to CW 52	51 times 7 equals 357
CW 53	31/12

When are there 54 calendar weeks?

54 Calendar weeks can only occur in leap years in the North American numbering of weeks, and this only happens about every 100 years. If the 1st of January in a leap year falls on a Saturday, for example, this day forms the 1st calendar week. This is followed by another 52 weeks. A leap year consists of 366 days, i.e., 52 weeks and 2 days. So there is one day left, which in this example is Sunday, the 31st of December. This Sunday is then the beginning of the 54th week.

Example for 54 North American Calendar weeks at 366 days (leap year)
	Su	Mo	Tu	We	Th	Fr	Sa
CW 1							01/01
CW 2 to CW 53	52 times 7 equals 364
CW 54	31/12

All weeks 2023 and the current week