Dates

Week Number Calculator

ISO 8601 week number calculator with Monday-based weekday, calendar day of year, and week date.

Find the ISO 8601 week number for a calendar date. ISO weeks begin Monday, and week 1 is the week containing the year's first Thursday.

An ISO week-year can differ from the calendar year near January 1 and December 31. Other business calendars may use different week numbering.

Frequently asked questions

What is ISO week 1?

It is the Monday-based week containing the first Thursday of the ISO week-year.

Why can January 1 belong to the prior week-year?

Dates before the first ISO Thursday can be part of the final ISO week of the previous year.

Do all companies use ISO weeks?

No. Payroll, retail, and fiscal calendars may follow different rules.