Week Number Calculator
Determine ISO week numbers, US standard weeks, and full date ranges for any calendar day instantly.
Selected Date ISO Week
Week 0
Monday, Jan 1 – Sunday, Jan 7
ISO 8601 Week
Starts Monday (ISO standard used in EU & global tech).
US / Traditional Week
Starts Sunday (Standard calendar system in US & Canada).
Day of Year
Progress through the total calendar year.
ISO Week Range (Mon - Sun)
Weeks in Year
About Week Numbering Systems
The Week Number Calculator resolves common discrepancies between standard calendar formats across international project planning, supply chain logistics, and software development.
System Differences Explained
- ISO 8601 (International Standard): Weeks begin on Monday. Week 1 is defined as the week containing the first Thursday of the calendar year (or January 4th). Years can contain either 52 or 53 weeks.
- US / Traditional System: Weeks begin on Sunday. Week 1 starts on January 1st regardless of which day of the week it falls on.