ISO Week Number

How ISO week numbers work

• Every week starts on Monday and ends on Sunday.
• Week 1 is the week that contains the first Thursday of the year. Put another way, it is the week that contains January 4.
• A year has 52 or 53 weeks. 2026 has 53 and 2027 has 52.
• The first days of January can belong to the last week of the year before, and the last days of December to week 1 of the next year. For example, Friday, January 1, 2027 is in week 53 of 2026.

The short form 2026-W40-7 means year, week and day of the week (1 = Monday, 7 = Sunday). The year in it is the week's year, which can differ from the calendar year for those few days around New Year.

Some calendars in the US number weeks differently, starting week 1 on January 1 or on a Sunday. Those numbers can be one off from ISO weeks. Ask which system is in use if a week number comes from a schedule or a form.

Weeks of 2026

Weeks of 2027

The rule is checked against a separate implementation for every day from 1900 to 2100: see the method page.