Week Number Calculator
Calculate the ISO week-numbering year and week for a calendar date.
How to use
- Choose a date.
- Run the calculation.
- Read the ISO week year and W01-W53 number.
Capabilities and scope
ISO week numbering can cross calendar-year boundaries, so early-January and late-December dates may belong to a different week-numbering year.
How it works
ISO weeks start on Monday and assign a week to the year containing its Thursday. The implementation shifts the date to that week's Thursday and counts weeks from the start of the resulting week year.
Example
An early-January date can display the previous week-numbering year when it belongs to that year's final ISO week.
Useful for
- Label weekly reports
- Check ISO-week-based logs or schedules
- Validate year-end week boundaries
Before you use it
- The result may differ from Sunday-first US-style week numbers or an organization's custom week convention.
Standards and references
Frequently asked questions
Why can a January date belong to the previous week year?
ISO assigns the week by its Thursday rather than by the calendar year of every date in the week.
Can an ISO year have week 53?
Yes. Some years contain W53 under the ISO week rules.