How to use this tool
ISO weeks start on Monday. Week 1 is the week containing January 4, which is also the first Thursday of the year. Because a week belongs to the year containing its Thursday, dates near New Year can have an ISO week-year that differs from their calendar year. This tool shows both the week number and the week-year.
Try these values
January 1, 2021 belongs to ISO week 53 of 2020, running from Monday December 28, 2020 through Sunday January 3, 2021. January 4, 2021 starts ISO week 1 of 2021.
Keep the week-year with the number
“Week 1” alone is ambiguous near New Year. ISO assigns a whole Monday-to-Sunday week to the year containing its Thursday. Store or share the full result, such as 2020-W53, rather than attaching the calendar year by habit. The Monday and Sunday boundaries make it easier to check that a reporting period matches the dates in your spreadsheet or project plan.
Check the convention used by your calendar
Some calendars start weeks on Sunday, and some reporting systems call the week containing January 1 week one. Those systems can disagree with ISO 8601 without either being a calculation error. Ask which convention is required before comparing reports. This page deliberately uses one rule throughout instead of guessing a regional week number from your browser language.
Common questions
Why is January 1 in the previous week-year?
ISO assigns a whole Monday–Sunday week to the year containing its Thursday.
Can a year have 53 weeks?
Yes. Some ISO week-years have 53 weeks rather than 52.
Is this the same as a Sunday-start calendar?
No. Regional Sunday-start week numbering can give a different result. This calculator specifically uses ISO 8601.
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