Week Numbers 2025

What Are ISO Week Numbers?

An ISO week number is a simple way to name a week with a number from 1 to 52 or 53, instead of a range of dates. Saying "week 32" is shorter than "the week of August 3" and means the same thing to anyone using the standard. The rules come from ISO 8601, an international date standard, so a week number means the same thing worldwide.

How week numbering works

Two rules cover almost everything:

• Weeks start on Monday and end on Sunday.
• Week 1 is the week that holds the year's first Thursday — the same as the week that contains January 4.

Because of that first-Thursday rule, the first few days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. That's normal, not a mistake.

Why 2025 has 52 weeks

Most years, including 2025, have 52 ISO weeks. A year only gets a 53rd week when it starts on a Thursday (or a Wednesday in a leap year). 2025 starts on a Wednesday, so it has the usual 52.

Who uses week numbers?

• Payroll and finance — weekly and bi-weekly pay periods.
• Project and agile teams — sprints and delivery schedules often run by week number.
• Manufacturing and logistics — production and shipping plans use "CW" (calendar week) or "KW" in Europe.
• Schools and universities — term weeks and teaching schedules.