Calculating week numbers in 2020 or 2019 with the week number calculator

Updated on by Michael Mühl

With the week number calculator, you can calculate the corresponding week number (calender week) using a date. Conversely, you can use a week number to determine the start and end date of the week you are looking for. In addition to the international ISO 8601 calculation method, you can use other internationally relevant methods to calculate week numbers.

Tips for making entries in the week number calculator

Depending on the selection made in the input of the calculator, different input fields open, which we describe in the following.

Calculation of

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.

Date input

Day

Please enter the day, i.e., the day of the month for calculating the week number.

Month

Please enter the month for the week number calculation.

Year

Please enter the year for the week number.

Week number entries / calender week

Week number

Please enter the week number for the calculation of week start and week end.

Year

Please enter the year of the week number for the calculation of week start and week end.

Calculation method

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.

Calculation methods supported by the week number calculator

International calculation according to ISO

The international ISO 8601 definition of a week is mainly used in European and Asian countries to number weeks:

Weeks start on a Monday and the first week of the year includes 4 January. 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 containing at least four days of the new year.
  • The first calendar week of a year is the week that contains the first Thursday of January.

Two examples of the first week number according to ISO 8601

  • If 1 January falls on a Wednesday, the first calendar week begins on Monday, 30 December. 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 1 January falls on a Friday, the first calendar week starts on Monday, 4 January. This is because the previous week would have contained only three days of the new year (Friday, Saturday, Sunday)

North American calculation of week numbers

In the USA, Canada and most Latin American countries, Australia as well as in Japan, Israel, South Korea and others, a different system is used for numbering 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 1 January and ends on the following Saturday.
  • The last week of a year always begins on the last Sunday of the year and ends on 31 December.

Two examples

  • If 1 January falls on a Wednesday, the first week of the year starts with this Wednesday and consists of four days (Wednesday, Thursday, Friday and Saturday).
  • If 31 December falls on a Tuesday, the last week of the year begins on Sunday, 29 December 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, as 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, whereas the ISO calendar weeks always consist of whole weeks.

Arabic calculation of week numbers

In the Arab countries, the system for the numbering of weeks is similar to the North American calculation method just described. However, in contrast to this, Saturday, instead of Sunday, is the start of the week. Thus, the week ends on Friday. Apart from that, the first calendar week always starts on 1 January here as well, regardless of the weekday.

In other words:

  • The first calendar week of a year always starts on 1 January 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 31 December.

Two examples

  • If 1 January falls on a Wednesday, the first calendar week starts with this Wednesday and consists of three days (Wednesday, Thursday and Friday).
  • If 31 December falls on a Tuesday, the last calendar week begins on Saturday, 28 December and consists of four days (Saturday, Sunday, Monday and Tuesday).

With the Arabic calculation method as well, the rule that the first week always begins with 1 January is more intuitive than with 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.

Different 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 financial year, which differs from the calendar year.

In the USA, the week starts predominantly with Sunday, but sometimes the start of the first week of the year is defined as in the ISO 8601 definition, i.e., not on 1 January.

Since the numbering of calendar weeks is not very common in the USA, 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 1 January. The weeks are then numbered sequentially, with each following week starting on Sunday.

More online calculators

Source information

As source for the information in the "Week Numbers" category, we have used in particular:

Last update on October 20, 2020

The pages of the "Week Numbers" category were last editorially reviewed by Michael Mühl on October 20, 2020. They all correspond to the current status.

Previous changes on June 25, 2020

  • Publication of the Week Number Calculator
  • Creation of the texts for the corresponding pages
  • Editorial revision of all texts in this category