About week number calculator
The ISO 8601 week system numbers weeks from 1 to 52 or 53, with each week starting on Monday. It is used in business planning and in many countries' calendars.
Rules
- Week 1 is the week that contains the first Thursday of the year.
- Early January days can belong to week 52 or 53 of the previous year, and late December days to week 1 of the next.
- Other systems, such as the US one, start weeks on Sunday and give different numbers.
A worked example
1 January 2026 is a Thursday, so it falls in ISO week 1 of 2026, which begins on Monday 29 December 2025. For 2025, week 1 began on Monday 30 December 2024. A year has 52 or 53 ISO weeks.
Why companies use week numbers
Production, shipping and retail plans often use week numbers because they do not depend on the varying lengths of months.
