Week Number Calculator
Find the ISO week number, ISO week-year and weekday for a calendar date.
Result for the values shown.
Calculations run in your browser.
What to enter
| Input | Meaning and units |
|---|---|
| Calendar date | Gregorian calendar date in YYYY-MM-DD format; years 0001 through 9999. |
Understanding your result
Calendar results use the stated counting and anniversary convention. Complete months and years differ from fixed day counts.
Common mistakes
- Counting both end dates when the method excludes a boundary, or assuming all months have equal length.
- Assuming the ISO week-year always equals the calendar year.
Check your calculation
- Test a short known interval and a leap-day boundary; for working days, verify your holiday list.
Calculation checks, sources and review limits
What ISO week number and week-year belong to this date?
Find the ISO week number, ISO week-year and weekday for a calendar date.
Common uses
- Find the ISO week number, ISO week-year and weekday for a calendar date.
How it works
ISO week starts Monday; week 1 contains Jan4; derive week-year from Thursday. Calendar arithmetic uses the proleptic Gregorian calendar. Explicit calendar dates are independent of local daylight-saving changes. Month/year additions clamp to the last valid day of the destination month. Timestamp arithmetic requires a stated UTC offset and excludes leap seconds.
Worked example
Enter Calendar date: 2021-01-01. The result is Week year: 2020; Week: 53; Weekday: 5.
FAQ
What ISO week number and week-year belong to this date?
ISO week starts Monday; week 1 contains Jan4; derive week-year from Thursday. Calendar arithmetic uses the proleptic Gregorian calendar. Explicit calendar dates are independent of local daylight-saving changes. Month/year additions clamp to the last valid day of the destination month. Timestamp arithmetic requires a stated UTC offset and excludes leap seconds.
What assumptions and limits apply?
ISO week-year can differ from calendar year; show both.