Please select the value that you want to calculate.
After selecting "Week number", the week number is calculated for the entered date.
After selecting the "start and end of a week number" the start and end date for the entered week number are calculated.
Please enter the day, i.e., the day of the month for calculating the week number.
Please enter the year of the week number for the calculation of week start and week end.
Select the method of week number calculation.
Available are "International according to ISO", "North American calculation" and the "Arabic calculation", which are described
in detail below.
Calculator ↑Contents ↑International calculation according to ISO
The international ISO 8601 definition of a week is used mainly in European and Asian countries for numbering weeks:
Weeks start on a Monday, and the first week of the year includes January 4. This means that up to three days from the previous year may be included in the first week of the current year, and up to three days from the current year may be included in the last week of the previous year.
In other words:
- The first calendar week of a year is the week that contains at least four days of the new year.
- The first calendar week of a year is the week that contains the first Thursday in January.
Two examples of the first week number according to ISO 8601
- If January 1 falls on a Wednesday, the first calendar week begins on Monday, December 30. This first week then contains two days of the old year (Monday and Tuesday) and five days of the new year (Wednesday to Sunday).
- However, if January1 falls on a Friday, the first calendar week begins on Monday, January 4. This is because the previous week would have contained only three days of the new year (Friday, Saturday and Sunday).
North American calculation of week numbers
In the USA, Canada and most Latin American countries, as well as Australia, Japan, Israel, South Korea and other countries, a different system is used for numbering the weeks.
In contrast to the ISO standard, Sunday is the first day of the week, which then ends with Saturday. In addition, the first calendar week – regardless of the day of the week – always begins on 1 January.
In other words:
- The first week of a year always starts on January 1 and ends on the following Saturday.
- The last week of a year always begins on the last Sunday of the year and ends on December 31.
Examples
- If January 1 falls on a Wednesday, the first week of the year begins on that Wednesday and consists of four days (Wednesday, Thursday, Friday and Saturday).
- If December 31 falls on a Tuesday, the last week of the year begins on Sunday, December 29 and consists of three days (Sunday, Monday and Tuesday).
The rule that the first week always begins on January 1 is more intuitive than in the ISO standard, since the last calendar week always ends with the final week of the year, and the first week always begins with the new year. However, at the turn of the year, there are usually partial weeks of less than seven days, while the ISO calendar weeks always consist of whole weeks.
Arabic calculation of week numbers
In Arab countries, the system of week numbering is similar to the North American method of calculation just described. Unlike the latter, however, the week does not begin with Sunday, but with Saturday. Thus, the week ends on Friday. Apart from that, the first calendar week always starts on January 1 here as well, regardless of the day of the week.
In other words:
- The first calendar week of a year always starts on January 1 and ends on the following Friday.
- The last calendar week of a year always begins on the last Saturday of the year and ends on December 31.
Examples
- If January 1 falls on a Wednesday, the first calendar week starts with that Wednesday and consists of three days (Wednesday, Thursday and Friday).
- If December 31 falls on a Tuesday, the last calendar week begins on Saturday, December 28, and consists of four days (Saturday, Sunday, Monday and Tuesday).
Also, in the Arabic method of calculation, the rule that the first week always begins with January 1 is more intuitive than in the ISO standard, where the last week always ends in the current year and the first week begins with the new year. Likewise, at the turn of the year, there are usually partial weeks of less than seven days, while the ISO calendar weeks continue to consist of whole weeks.
Other definitions
There are many other definitions for numbering weeks.
In some companies, the first week begins in the middle of the year in order to accurately reflect their fiscal year, which differs from the calendar year.
In the U.S., the week predominantly begins on Sunday, but sometimes the beginning of the first week of the year is defined as in the ISO 8601 definition, i.e., not January 1.
Since the numbering of calendar weeks is not very common in the U.S., the counting method used in the calculator was chosen to reflect the way people speak in casual conversation:
"Week one is the first week of the year, and it starts on January 1. The weeks are then numbered consecutively, with each successive week starting on Sunday.
