How to use this week number calculator
Choose a date. The result updates immediately and reports its ISO week as year-Wweek, plus the weekday where Monday is 1 and Sunday is 7. This tool uses the international ISO convention rather than a locale-specific Sunday-first system.
How the calculation works
ISO weeks begin on Monday. Week 1 is the week containing January 4, equivalently the week containing the year's first Thursday. Most ISO week years contain 52 weeks, while some contain 53.
The ISO week-based year can differ from the calendar year around late December and early January. Reporting both values prevents a date such as January 1 from being assigned to the wrong reporting period.
Formulas
ISO Week 1
ISO weekday
Worked examples
January 1, 2021
January 1, 2021 falls in ISO week 2020-W53 because its Thursday belongs to the previous week-based year.
January 4, 2021
January 4 is Monday of ISO week 2021-W01.
Common uses
Business reporting
Assign transactions, activity, or performance records to consistent Monday-to-Sunday ISO reporting weeks, especially around year boundaries where the week-based year can differ from the calendar year.
Project schedules
Reference milestones by standardized week number across teams and countries. Include the ISO year as well as the week number to avoid ambiguity.
Calendar and software checks
Verify week labels produced by calendars, spreadsheets, databases, and scheduling code against the ISO first-Thursday convention.
Accuracy and limitations
The calculator uses Gregorian dates and the ISO 8601 Monday-first, first-Thursday rule. It reports the ISO week-based year separately from the calendar year.
Other systems may start weeks on Sunday or define week 1 differently. Confirm the convention required by a workplace, locale, spreadsheet function, or reporting standard.
Frequently asked questions
When does an ISO week begin?
Monday is the first day and Sunday is the seventh day.
Why can the week year differ from the calendar year?
ISO assigns an entire Monday-to-Sunday week to the year containing that week's Thursday.
Can a year have week 53?
Yes. A week-based year has 53 weeks when the calendar alignment supplies an extra Thursday-containing week.
Is this the same as every calendar app?
No. Some locales number Sunday-first weeks differently. This calculator explicitly uses ISO 8601.
Is the week number calculator free to use?
Yes. NumUtility is free, requires no account, and places no limit on ordinary calculations.
Are my entries stored?
No. The calculation runs in your browser. Values entered into the tool are not sent to NumUtility or saved by us.