How to Use

1
Choose a Date from the calendar picker.
2
Click Calculate Week to get results.
3
Review the ISO week, US week, day of year, quarter, and weekday. Use Reset for additional dates.

How It Works

ISO 8601 Week

Weeks start Monday; week 1 contains Jan 4

US Week

Weeks start Sunday; week 1 contains Jan 1

Day of Year

Days elapsed since January 1 (inclusive)

Examples

Release planning

Map launch dates to ISO sprint weeks.

Result: easier cross-team cadence alignment.

Finance reporting

Group entries by week and quarter.

Result: cleaner period summaries.

Operations calendars

Validate week labels for shift schedules.

Result: consistent weekly naming.

FAQ

ISO 8601 weeks start on Monday and week 1 must contain January 4th. US weeks start on Sunday and week 1 contains January 1st. This causes different numbering, especially at year boundaries.

Yes. Under ISO 8601 rules, dates like December 29-31 can fall into week 1 of the following year, and January 1-3 can belong to week 52 or 53 of the previous year.

Most years have 52 ISO weeks, but some years (called "long years") have 53 weeks. This happens roughly every 5-6 years.