Week Number Calculator
The ISO 8601 and US week number for any date, with the week's date range and the year's week count.
What Is the Week Number Calculator?
The Week Number Calculator reports both common conventions for any date: the ISO 8601 week (Monday start, week 1 contains the year's first Thursday — the standard in Europe and business systems) and the US convention (Sunday start, week 1 begins January 1). It also shows the exact Monday–Sunday range of the ISO week and whether the ISO year has 52 or 53 weeks.
How It Works
Pick a date (or press Today). Both week numbers and the week's date range update instantly.
When to Use It
Use it for planning against European schedules, manufacturing and logistics calendars, sprint numbering, or any 'which week is it?' question.
Frequently Asked Questions
- Why Do The Two Numbers Differ?
- They follow different rules: ISO weeks can assign late December to week 1 of the next year, while the US count restarts exactly on January 1.
- When Does A Year Have 53 ISO Weeks?
- When January 1 falls on a Thursday — or on a Wednesday in a leap year.
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