What is Week Number?
ISO week numbers are used in planning, reporting and logistics. This calculator shows the ISO week-year, week number, weekday and day of year.
How to use
- Choose a date.
- Read the ISO week number and ISO week-year.
- Use day-of-year for logs, reports and ordinal date references.
Example
Use this sample to confirm what Week Number does before pasting your own data.
Input
2026-06-29
Output
ISO week number, ISO weekday and day-of-year.
When to use it
Use Week Number for quick local checks, debugging, documentation prep, and one-off conversions where opening a full IDE or command-line workflow would be slower. For production-critical data, always review the output before committing or shipping it.
Frequently asked questions
Why can ISO week-year differ from calendar year?
Early January dates can belong to the last ISO week of the previous year, and late December dates can belong to week 1 of the next ISO year.
Which day starts the ISO week?
Monday is ISO weekday 1.