How to use this calculator
- Enter or choose date.
- The day of the week updates as you type. Use Copy result or Share to keep it.
Method
Count the days from a reference date of known weekday (1 January 1970 was a Thursday) and take the remainder after dividing by 7. ISO week = the week (Monday to Sunday) containing that week’s Thursday, numbered from the first week with a Thursday in the year.
- Leap year
- divisible by 4, except century years not divisible by 400
Worked example
- 20 July 1969 is 165 days before 1 January 1970 (a Thursday).
- 165 = 23 × 7 + 4, so the date falls 4 weekdays before Thursday: Sunday.
- Day of the year: 31 + 28 + 31 + 30 + 31 + 30 + 20 = 201.
20 July 1969, the day of the Apollo 11 Moon landing, was a Sunday.
What the result means
ISO week numbers are used in business planning and in much of Europe; a few days at the start or end of a year can belong to the neighbouring year’s week 1 or week 52/53.
Assumptions and limits
- The proleptic Gregorian calendar is used for all dates.
- Historical dates before a country adopted the Gregorian calendar (1582 in much of Catholic Europe, 1752 in Britain and its colonies) may have been recorded in the Julian calendar, with a different weekday mapping.
Questions
Why can 30 December be in week 1?
ISO 8601 assigns each Monday-to-Sunday week to the year that contains its Thursday. 30 December 2024 was a Monday, and that week’s Thursday was 2 January 2025, so it is 2025-W01.
Is 2000 a leap year?
Yes. Century years are leap years only if divisible by 400, so 2000 was a leap year but 1900 was not.