Week Number Calculator
October 12, 2026 falls in ISO week 42 of 2026 — day 285 of 365.
Pick a date
About this calculation
Enter any date to get its ISO-8601 week number, the day of the year, the quarter and how many weeks are left in the year.
ISO-8601 defines week 1 as the week containing the first Thursday of the year, and treats Monday as the first day of the week. This is the standard used in most of Europe, in international business planning and in payroll systems.
Note that ISO weeks can differ from the simple "week 1 starts on 1 January" convention — particularly around New Year, when a date in early January can belong to week 52 or 53 of the previous year.
Frequently asked questions
What standard is used?
ISO-8601: weeks start on Monday, and week 1 is the week containing the first Thursday of the year.
Why can 1 January be in week 52?
Because ISO weeks are determined by Thursdays. If 1 January falls on a Friday, Saturday or Sunday, it belongs to the final week of the previous year.
How many weeks are in a year?
Usually 52, occasionally 53. A year has 53 ISO weeks when it starts on a Thursday, or is a leap year starting on a Wednesday.
Is this the same as US week numbering?
No. The United States commonly uses a Sunday-start convention, which can produce a different week number for the same date.