Calculator
ISO 8601 defines a whole alternate way to write a date — year, week number, and weekday, like 2026-W30-4. Convert either direction below.
An ISO week date has three parts: the ISO week-numbering year (which can differ from the calendar year for a few days in early January or late December), the week number (01–52 or 53, prefixed with W), and the day of the week (1 for Monday through 7 for Sunday). 2026-W30-4 means: ISO week-numbering year 2026, week 30, Thursday.
ISO week dates are common in manufacturing date codes, European payroll and scheduling systems, and some project-management tools that plan by week number rather than by calendar date. For just the week number without the full week-date string, the Week Number Calculator is simpler.
Related
Just the ISO week number for a date, without the full format.
ISO 8601, US, EU, RFC 2822 and more, side by side.
What day of the week any date falls on.