Week Number Calculator

About ISO Week Numbers

The ISO 8601 standard defines a week numbering system. ISO weeks start on Monday. Week 1 of a year is the week containing the first Thursday of the year. A year has either 52 or 53 weeks.

The day of the year (DOY) tells you how many days have passed since January 1st. It ranges from 1 to 365 (or 366 in a leap year).