Week Number Calculator (ISO 8601 Week of the Year)
Find the ISO 8601 week number for any date, plus the ISO week-year, the day of the year, and the calendar quarter.
Week 1 holds the first Thursday.
Which week of the year is it?
Business, payroll, manufacturing, and project schedules in much of Europe and beyond track time by week number rather than by date, as in “ship in week 32.” The international standard for this is ISO 8601, and it has a precise definition that does not always match a naive “divide the day of the year by seven.”
The ISO 8601 rules
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year’s first Thursday. Equivalently, it is the week containing January 4.
- A year has either 52 or 53 weeks.
The consequence is that the first few days of January can belong to the last week (52 or 53) of the previous year, and the last few days of December can belong to week 1 of the next year. That is why ISO defines a separate “week-year” that can differ from the calendar year. For example, January 1, 2023 was a Sunday, so it fell in week 52 of week-year 2022.
What this calculator returns
For any date it gives the ISO week number, the ISO week-year (which may not equal the calendar year near the new year), the day of the year (1 to 365 or 366), and the calendar quarter.
Worked example
December 31, 2024 is a Tuesday. Its week contains January 1, 2025, whose first Thursday is January 2, so that Tuesday already belongs to week 1 of week-year 2025, even though the calendar still says 2024.
Where it shows up
European business calendars, retail and supply-chain planning, fiscal week reporting, and software date handling, where mixing up the calendar year and the ISO week-year is a classic bug.