Week Number Calculator

Choose a date to calculate its ISO week number and week-year.

Date

—ISO week number

Core timing workflows

Start with a direct timer, stopwatch, alarm or focus workflow. Calendar, date arithmetic, clocks and scheduling specialists remain available from the full Date & Time hub.

All 76 date & time tools

Online TimerCountdown, sound, repeat & fullscreen StopwatchLaps, precision & fullscreen Countdown TimerFocused countdown workflow Alarm ClockAlarm at a local clock time PomodoroFocus and break cycles Interval TimerWork/rest rounds Presentation TimerTalk timing with warnings Classroom TimerProjector-friendly countdown

ISO week in both directions with boundary evidence

Resolve a date to its ISO week-year and Monday–Sunday range, then reverse an ISO year/week back to all seven calendar dates.

DateReverse ISO yearReverse week

—ISO week

—Weekday

—Week range

—Weeks in ISO year

Ready.

ISO weekday	Weekday	Date

Calendar or duration rule

ISO week dates use Monday-start weeks and week 1 contains the year’s first Thursday.

Dates near New Year can belong to the adjacent ISO week-year.

Worked check

ISO week numbering can cross calendar-year boundaries. For example, January 1, 2021 belongs to ISO week 53 of ISO week-year 2020. Do not assume the ISO week-year always matches the ordinary calendar year shown in the date.

Rule: ISO week dates use Monday-start weeks and week 1 contains the year’s first Thursday. Dates near New Year can belong to the adjacent ISO week-year. For a final check, distinguish calendar arithmetic from elapsed duration, confirm the intended local date or time zone, and inspect month-end, leap-year or daylight-saving boundaries when they can affect the meaning.

ISO week-date semantics

ISO weeks start Monday. Week 1 is the week containing January 4, so late-December dates can belong to the next ISO week-year and early-January dates can belong to the previous one.