January can belong to the previous week year
January 1, 2021 is 2020-W53-5: Friday in week 53 of ISO week year 2020. That week runs from December 28, 2020 through January 3, 2021.
Calendar date: 2021-01-01
2020ISO week-numbering year
53Week of 53
FridayISO weekday 5
Monday 2020-12-28 → Sunday 2021-01-03. Week 1 contains January 4. The week-numbering year can differ from the calendar year.
Calculations run in your browser. Dates and settings are not uploaded by this tool.
How to use the ISO Week Number Calculator
- 01
Choose a calendar-date lookup or a reverse ISO week-date conversion.
- 02
For a reverse conversion, enter the ISO week year, week number, and weekday, where Monday is 1 and Sunday is 7.
- 03
Read the full YYYY-Www-D result and the Monday–Sunday range. Use the week-numbering year when copying the result into another system.
Examples
December can belong to the next week year
December 31, 2018 is 2019-W01-1. It is Monday of the first ISO week of 2019, even though its calendar year is still 2018.
How ISO week 1 is chosen
An ISO week starts on Monday. Week 1 contains January 4, which is equivalent to containing the first Thursday of the calendar year. The Thursday of any week determines that week’s ISO year. This definition keeps complete seven-day weeks together instead of starting a new partial week on January 1.
Week 53 is a real constraint
An ISO week year has either 52 or 53 weeks. The reverse converter checks the selected year before accepting week 53; it does not roll an invalid week into the following year. When importing a week label from a spreadsheet or planning system, preserve all three parts: the week year, week number, and weekday.
Other week conventions can give different answers
A Sunday-based calendar, a system that labels January 1 as week 1, and an ISO calendar can disagree for the same date. This calculator uses ISO rules only and labels that choice in the result. If a workplace uses a fiscal week system or a retail calendar, its labels require that organization’s separate year-start and week rules.
Common questions
Why is the year different from my input date?
Dates near January 1 or December 31 can sit in a complete week assigned to the neighboring ISO year. The calendar date remains unchanged.
Is Sunday weekday 0 or 7?
ISO weekday numbering uses Monday 1 through Sunday 7. The reverse converter follows that convention explicitly.
Can I reverse any week date?
The week must exist in the selected ISO year, and the resulting calendar date must fall within years 0001–9999. At the supported range edge, part of a displayed week can lie beyond that boundary.