Week Number Calculator
Find the ISO week number for any date, convert an ISO week number to its Monday-to-Sunday date range, and compare with a Sunday-start US week number.
Find the ISO week number
The main English intent is ISO week lookup: choose a date and get the ISO 8601 week number, week-year and Monday-to-Sunday range.
ISO weeks start on Monday. Dates near January 1 can belong to the previous or next ISO week-year.
Date to week and week to dates
- Date-to-week mode returns ISO week number, ISO week-year and full week range.
- Week-to-dates mode converts an ISO week number and week-year to Monday and Sunday dates.
- A supporting US week number is shown for Sunday-start calendar contexts.
- The calculator validates whether the selected year has 52 or 53 ISO weeks.
Frequently Asked Questions
Sources and References
- DateMDN Web Docs
- Representing dates and timesMDN Web Docs
- Gregorian calendarBritannica
- ISO 8601 date and time formatISO
- International System of Units: TimeNIST
Calculations are based on the listed reference sources. Links open in a new tab.
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