How ISO week numbers work
ISO 8601 weeks start on Monday. Week 1 is the week that contains the year's first Thursday, which is the same as the week containing 4 January. So a year has 52 or 53 weeks, and the first days of January can belong to the last week of the previous year.
Many countries, planning tools and weekly launch lists, including Aback Launch's, use ISO weeks, which is why the same date can be week 1 in one calendar app and week 53 in another set to US conventions.
Questions
What week is it today?
The calculator at the top of this page shows today's ISO week number and its Monday to Sunday dates, from your device's clock.
Which years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or is a leap year starting on a Wednesday. 2026 starts on a Thursday, so it has 53.
Why does my calendar show a different week number?
Some calendars, common in the US, start weeks on Sunday and count week 1 from 1 January. ISO weeks start on Monday and use the first Thursday rule.
Does the time zone matter?
Only around midnight on Sunday night. The calculator uses your device's date.