What is an ISO week number?

ISO week numbers follow the ISO 8601 standard, where week 1 is the first week with at least 4 days in the new year, and weeks start on Monday.

Why doesn't January 1st always start week 1?

Under ISO 8601, if January 1st falls late in the week, those days may belong to the last week of the previous year.

Are there other week numbering systems?

Yes, some systems start weeks on Sunday or define week 1 differently. This calculator uses the internationally recognized ISO 8601 standard.

Week Number Calculator

Overview

Easily determine the week number for any date, ideal for project timelines, financial planning, and academic calendars. Perfect for staying organized.

The Week Number Calculator allows you to find the specific week number for any given date. This is particularly useful for project planning, fiscal calendars, or academic schedules where week numbers are commonly referenced.

How to use

Find the week number in a few simple steps.

Enter a date: Input the date you want to check.
Calculate: Click calculate to find the week number.
View week number: Review the ISO week number and year.

FAQs

What is an ISO week number?: ISO week numbers follow the ISO 8601 standard, where week 1 is the first week with at least 4 days in the new year, and weeks start on Monday.
Why doesn't January 1st always start week 1?: Under ISO 8601, if January 1st falls late in the week, those days may belong to the last week of the previous year.
Are there other week numbering systems?: Yes, some systems start weeks on Sunday or define week 1 differently. This calculator uses the internationally recognized ISO 8601 standard.