Week Number Calculator

ISO 8601 defines week numbers so that week 1 is the week containing the first Thursday of the year — which means early January dates can belong to the previous year's week 52 or 53, and late December dates can be week 1 of the next year.

ISO weeks start on Monday. This is the standard used in Europe for business reporting, manufacturing schedules, and payroll periods. US calendars often use Sunday-start weeks without ISO numbering — do not mix conventions in the same report.

Example: July 31, 2026 falls in ISO week 31 of 2026. January 1, 2026 is in week 1 of 2026 because that week contains the first Thursday (January 1, 2026 is a Thursday).

The first-Thursday rule has an equivalent form that is easier to reason about: ISO week 1 is the week containing January 4, and it is also the first week with at least four days in the new year. Both statements describe the same rule, which exists so that every week belongs entirely to one year rather than being split across two.

Most ISO years have 52 weeks, but some have 53. A year gets a 53rd week when it starts on a Thursday, or when it is a leap year starting on a Wednesday — 2026 is a 53-week year on this rule. Reporting systems that assume 52 weeks silently drop or duplicate a period when a 53-week year arrives, which is a recurring source of year-end reconciliation errors.

Because weeks are indivisible, the ISO week-year can differ from the calendar year for a few days at each boundary. December 31, 2019 sits in week 1 of ISO year 2020, and January 1, 2021 sits in week 53 of ISO year 2020. Always pair a week number with its week-year, since “week 1” alone is ambiguous across a new year.

The Monday start is part of the standard and is what separates ISO numbering from informal US usage. A Sunday-start week shifts every boundary by a day, so the same date can carry two different week numbers depending on convention — which is why mixing them in one report produces off-by-one comparisons that are very hard to spot.

Retail and payroll calendars often use neither. The 4-5-4 retail calendar common in US and Canadian retail defines its own period structure and a 53rd week roughly every five or six years, and many payroll systems key weeks to a fiscal year start instead. Confirm which calendar a report is built on before comparing week numbers across systems.

US retail fiscal calendars sometimes define week 1 differently from ISO — confirm whether your payroll or inventory report uses ISO, Sunday-start, or Saturday-start weeks before filing.