Week Number of a Date

ISO 8601 defines week 1 as the week containing the first Thursday of the year (equivalently, the week containing January 4). Weeks start on Monday. This is the standard used in Europe, most of Asia and international business reporting.

US convention starts weeks on Sunday and defines week 1 as the week containing January 1. Near year boundaries the two systems often disagree by one week — for example January 1, 2023 was ISO week 52 of 2022 but US week 1 of 2023.

Under ISO 8601, dates in late December can belong to week 1 of the following year, and dates in early January can belong to week 52 or 53 of the previous year. The 'YearISO' field shows the correct ISO year for the displayed week, which may differ from the calendar year.

Most years have 52 ISO weeks. Years that start on a Thursday (or leap years that start on a Wednesday) have 53 weeks — for example 2020, 2026, 2032. The US convention always assigns 53 weeks if January 1 is the only day of week 1.

Use ISO 8601 for international communication, EU business, software development and any situation where unambiguous week dating matters. Use US convention only when working specifically with US-based systems that use it (some payroll, retail and broadcast scheduling).

Weeks go up to 52 in most years and 53 in long years (those starting on Thursday or Wednesday in leap years). There is never a week 54 in either system.