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ISO week calculator

Convert a calendar date to an ISO week date, or find the date for a numbered week.

Your ISO week2020-W53-5
Calendar date2021-01-01
Week-year2020
Monday2020-12-28
Sunday2021-01-03
YYYY-MM-DD
Processed on your device

How it works & useful details

How it works & limits

ISO weeks start on Monday. Week 1 contains January 4, so dates near New Year can belong to the previous or following week-year. For example, January 1, 2021 is 2020-W53-5.

Select any day in the week strip to explore its calendar date. Only valid weeks are accepted: not every year has week 53. The complete week must fit inside years 0001–9999.

Your working space

Explore the live preview, then save a copyable text report. Inputs stay on this device and are cleared by refreshing. Dates use the Gregorian calendar, including before its historical introduction.