Understanding the result
The result gives the ISO week number (1 to 53) and the ISO week-year it belongs to, also written in the compact form 2026-W38. Below are the Monday and Sunday that bound the week and the number of weeks in that week-year.
Near the start and end of a year, the week-year can differ from the calendar year of the date, and the tool points this out when it happens.
What is an ISO week number?
ISO 8601 defines a standard way to number weeks. Weeks run from Monday to Sunday, and week 1 of a year is the week that contains the first Thursday of that year (equivalently, the week containing 4 January).
It is widely used in Europe and in business planning, logistics and software, wherever "week 32" needs to mean the same week to everyone.
How it works
For any date, find the Thursday of the same Monday-to-Sunday week. The calendar year of that Thursday is the ISO week-year. The week number is how many whole weeks separate that Thursday from the start of its year, plus one.
A week-year has 52 or 53 weeks. It has 53 when 1 January is a Thursday, or when it is a Wednesday in a leap year. 28 December always falls in the last week of its week-year.
Examples
1 January 2021 (a Friday)
ISO week 53 of 2020. Its Thursday is 31 December 2020, so the week belongs to 2020. Week 1 of 2021 begins on Monday 4 January.
31 December 2024 (a Tuesday)
ISO week 1 of 2025. Its Thursday is 2 January 2025, so the week belongs to 2025 even though the date is in 2024.
1 January 2026 (a Thursday)
ISO week 1 of 2026. 2026 also has 53 ISO weeks because it starts on a Thursday.
Things to keep in mind
- ISO weeks are not the same as the week numbers in some other systems. In the United States, Canada and some other countries, weeks commonly start on Sunday and week 1 is the week containing 1 January, so the numbers can differ by one.
- Use the ISO week-year, not the calendar year, when writing a week reference such as 2020-W53.
- The calendar tool can show these week numbers beside each row of a month.