How ISO week numbers work
Under the ISO 8601 standard, weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4). That means a year has either 52 or 53 ISO weeks, and the first days of January can belong to week 52 or 53 of the previous year – while the last days of December can be in week 1 of the next year. ISO weeks are widely used in Europe, in manufacturing, logistics, retail and payroll calendars.
US week numbers
In the United States, calendars and many spreadsheets start the week on Sunday and call the week containing January 1 “week 1”. The two systems can differ by one, so the calculator shows both. In Excel, =ISOWEEKNUM(A1) gives the ISO week and =WEEKNUM(A1) gives the US-style week.
Planning around working days? Try the business days calculator.
Frequently asked questions
How many weeks are in a year?
Most years have 52 ISO weeks. Years that start on a Thursday (or leap years starting on a Wednesday) have 53 weeks – for example 2026 has 53 ISO weeks.
Does the week start on Monday or Sunday?
ISO 8601 and most of Europe start on Monday. The US, Canada and Japan commonly start calendars on Sunday.
What day of the year is it?
The calculator shows the day of the year (1–366) and how many days are left in the year for the selected date.