ISO Week Date Converter (YYYY-Www-D)
The ISO week date is the ISO 8601 representation `YYYY-Www-D` that identifies a date by its ISO year, ISO week number, and weekday — e.g. `2026-W33-4` means Thursday of week 33 of 2026. It exists because calendar dates alone do not tell you which week a date falls in.
ISO weeks run Monday to Sunday; week 01 is the week containing the first Thursday of the year. This converter takes any date and returns its ISO week-date string plus the underlying week number and ISO year, making planning and reporting by week trivial.
ISO Week Date Converter (YYYY-Www-D) Quick Reference
| Format | Pattern / Description | Example 1 | Example 2 |
|---|---|---|---|
| ISO Week Date | YYYY-Www-D | 2026-W33-4 | 2026-W01-4 |
* Examples are rendered in UTC for the fixed sample instants above.
When to Use ISO Week Date (YYYY-Www-D)
Production & manufacturing planning
Schedules expressed as ISO weeks (W33, W34…) avoid ambiguity about which week a task belongs to, especially across year boundaries.
Financial & retail reporting
Weekly sales, payroll and inventory reports keyed by ISO week number align perfectly with Monday-start weeks used across Europe and Asia.
Calendar and scheduling apps
When your app displays "Week 33" or accepts `2026-W33` input, this converter verifies the exact date range and weekday mapping.
Frequently Asked Questions
What is the ISO week date format?
It is `YYYY-Www-D`, e.g. `2026-W33-4` = Thursday of week 33. `W` is a literal prefix, `ww` is a two-digit week number, `D` is the weekday (1 = Monday … 7 = Sunday).
What is ISO week number 01?
Week 01 is the week containing the first Thursday of the year. This rule keeps every ISO year at 52 or 53 weeks and can make the ISO year differ from the calendar year at the boundaries.
Why does the ISO week start on Monday?
The ISO 8601 standard defines the week as Monday to Sunday for business and industrial purposes. This differs from US convention (Sunday start) but is the worldwide standard.
How many ISO weeks are in a year?
Usually 52, but years that begin on Thursday (or leap years beginning Wednesday) have 53 weeks. 2026 has 53 ISO weeks.
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