Week Number | How to use and interpret results
Find a Monday-start week number, its week-year, and the Monday/Sunday boundaries. Around New Year, a date's calendar year can differ from the year assigned to its week.
- Runs in your browser
- Nothing is uploaded
- No signup; device limits apply
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How to use this tool
Select a date or use today. Before comparing with a calendar that starts on Sunday, check that the week-start convention matches.
Results and examples
2021-01-01 belongs to week 53 of 2020 under the Thursday-based week-year rule. Its Monday is 2020-12-28 and Sunday is 2021-01-03, crossing calendar years.
Worked example
2021-01-01 belongs to week 53 of 2020 under the Thursday-based week-year rule. Its Monday is 2020-12-28 and Sunday is 2021-01-03, crossing calendar years.
Limitations
- Fiscal-year, company-specific and Sunday-start numbering are not selectable.
- Local date handling makes boundary checks worthwhile when comparing external systems.
Fields
| Field | Guidance and constraints |
|---|---|
| Date | Check the unit and format displayed beside this field. |
| Summary | Check the unit and format displayed beside this field. |
Data handling
Processing stays in the current browser tab; the site does not upload tool input for the calculation itself.
Sources
Frequently asked questions
Why can January 1 belong to the previous week-year?
2021-01-01 belongs to week 53 of 2020 under the Thursday-based week-year rule. Its Monday is 2020-12-28 and Sunday is 2021-01-03, crossing calendar years.
Will it match my employer's week numbers?
Fiscal-year, company-specific and Sunday-start numbering are not selectable. Local date handling makes boundary checks worthwhile when comparing external systems.
