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Week number calculator (ISO 8601)
Enter any date and the calculator instantly returns its ISO 8601 week number. The ISO standard defines Week 1 as the week containing the first Thursday of the year, with each week starting on Monday. The result also shows the ISO year — which may differ from the calendar year near year boundaries — and the ISO day of the week.
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How we calculate the ISO week number
The algorithm determines the ISO day of the week (1 = Mon, 7 = Sun), then shifts the date to the nearest Thursday (the Thursday "anchors" the week in the ISO standard). The ISO year is derived from that Thursday. The week number is the count of weeks between Monday of Week 1 and the Monday of the week containing the Thursday, plus 1.
Example: 10 June 2026
10 June 2026 is a Wednesday (ISO day 3). Nearest Thursday: 11 June. Thursday 11 June falls in week 24 of year 2026. Result: week 24, ISO year 2026, day of week 3 (Wednesday).
Frequently asked questions
What is the ISO 8601 week number?
The ISO 8601 week number is a standardised way of numbering weeks. Week 1 is the week containing the first Thursday of the year. Weeks always start on Monday, and a year has 52 or 53 weeks.
When does Week 1 start in a given year?
Week 1 is the week containing 4 January, or equivalently the first Thursday of the year. It can start as early as 29 December of the previous year if 1 January falls on a Friday, Saturday or Sunday.
Is 1 January always in Week 1?
No. If 1 January falls on a Friday, Saturday or Sunday it belongs to the last week (52nd or 53rd) of the previous ISO year. For example, 1 January 2016 (Friday) belonged to ISO Week 53 of 2015.
Can a year have 53 weeks?
Yes. A year has 53 weeks when 1 January falls on a Thursday, or when it is a leap year and 1 January falls on a Wednesday. About 71 out of every 400 years have 53 weeks.
How do I manually calculate the ISO week number?
Find the nearest Thursday to your date. Then count how many full weeks lie between the Monday of Week 1 and the Monday of your Thursday's week, and add 1. The calculator does this automatically.
What is the ISO week number used for in practice?
Week numbers are widely used in production planning, logistics ("delivery in Week 24"), project scheduling, invoicing of recurring services, sales data analysis, and corporate or school calendars.
Can the ISO year differ from the calendar year?
Yes. A few days around the year boundary can belong to a different ISO year. For example, 31 December 2018 (Monday) belonged to ISO Week 1 of 2019. The calculator displays both the week number and the correct ISO year.
How do I know what day of the week a date falls on?
The calculator automatically determines the ISO day of the week (1 = Monday, 7 = Sunday) and displays it alongside the week number. Just enter the year, month and day.
Is the week number the same everywhere in the world?
ISO 8601 is used in Europe and many other countries, but the USA and Canada traditionally start the week on Sunday and define Week 1 as the week containing 1 January — giving different week numbers. This calculator uses ISO 8601 only.
How does the calculator handle leap years?
Leap years (366 days, February = 29 days) are fully supported. The ISO rule does not change — Thursday is still the "anchor" of the week. Enter any date in a leap year (e.g. 29 February 2024) and the result will be correct.
Results follow ISO 8601. The USA and Canada use a different week-numbering convention — results may differ.