Convert Gregorian dates to ISO week-year, week and weekday and back.
Type or paste on the left — the result updates as you type.
Weeks start Monday; week 1 contains January 4. Week-year may differ from calendar year. Invalid week 53 values are rejected.
Up to 100,000 UTF-16 units per input and 1,000,000 per result. See the supported formats and examples below.
Tool page updated: 2026-09-27
