Week Number Calculator

Look up a date in the ISO week calendar

Choose a date and select Find week number. The result shows its ISO 8601 week number, associated week-year, and the Monday-through-Sunday dates that form that week.

ISO weeks start on Monday

This calculator uses the widely used ISO week-date convention. Every week begins Monday and ends Sunday. Week 1 is the week containing January 4, equivalently the year's first Thursday. That precise rule avoids partial “week zero” periods, but it means the ISO week-year does not always match the calendar year printed in a date.

Why New Year's dates can surprise you

Late December can belong to week 1 of the next ISO year, while the first days of January can remain in week 52 or 53 of the previous ISO year. The displayed week-year makes this boundary explicit. A report labeled only “Week 1” is ambiguous unless its week-year is also recorded.

Worked example

January 1, 2021 was a Friday. The week containing it began Monday, December 28, 2020 and ended Sunday, January 3, 2021. Because that week contained the final Thursday of 2020 rather than January 4, the date belongs to ISO week 53 of ISO week-year 2020, not week 1 of 2021.

Common planning uses

Week numbers help label production schedules, sprint plans, timesheets, manufacturing batches, delivery windows, and recurring meeting notes. Always confirm which convention another organization uses. Some calendars start weeks on Sunday, define week 1 as the week containing January 1, or use fiscal periods that do not follow ISO rules.

Assumptions and limitations

The selected date is interpreted as a local calendar date without time, time zone, or daylight-saving conversion. Results use the proleptic Gregorian calendar supported by the browser. The page does not calculate payroll periods, academic terms, retail 4-4-5 calendars, or locale-specific numbering. Historical dates may predate formal adoption of the modern calendar or ISO standard even though the calculation can still extend the rule backward.

Week Number FAQ

Can an ISO year have 53 weeks?

Yes. Certain year-and-weekday combinations create a full week 53.

Is Sunday the first day?

No. ISO weeks run from Monday through Sunday.

Why is the week-year different?

The week crossing New Year's Day belongs to whichever year contains that week's Thursday.