Detailed guide
How to use the calendar-week calculator
How an ISO calendar week is determined
ISO 8601 weeks begin on Monday and end on Sunday. Week 1 is the week containing the first Thursday of the year, so it can start in late December. Conversely, the first days of January can still belong to the final ISO week of the previous week-numbering year.
The tool therefore displays the ISO year alongside the week number. That extra field prevents ambiguity around New Year, when the calendar year and week-numbering year can differ.
Calendar weeks in planning and communication
Week numbers are common in production, logistics, project planning and appointment coordination. “Week 1 of 2026” is more precise than “week 1” because the number repeats every year. The Monday and Sunday define the complete period.
Some countries and software settings use weeks that start on Sunday or define week 1 differently. ISO 8601 is widespread in European business, but international partners should still name the convention explicitly.
Understanding weeks at year boundaries
An ISO week-numbering year has 52 or 53 complete weeks. Because every week contains seven days, part of the first or last week often falls in an adjacent calendar year.
That overlap is expected, not a calculation error. Annual calendars on kalender.wiki also show ISO week numbers alongside month rows, while this calculator resolves one specific date in detail.