ISO Week vs US Week Numbering: What's the Difference?
ISO 8601 week numbers and US week numbers disagree by up to a full week. Here's why, and how to know which one your tools use.
Your finance system says week 1. Your project tool says week 52. You're not losing it. You're staring at the gap between ISO 8601 weeks and the US convention.
They start the year in different places, and they start the week on different days.
The ISO 8601 rule
ISO weeks start on Monday. Week 1 is the week containing the first Thursday of January, which is the same as saying the week containing January 4. So early January can sit in last year's W52 or W53, and late December can already be next year's W1.
The US rule
The US convention starts weeks on Sunday. Week 1 is simply whatever week January 1 lands in. Easier to picture. You just end up with partial weeks at both ends of the year.
When does it matter?
- European retail and logistics quote ISO weeks. "Ship in W34" almost always means ISO.
- US payroll, sports schedules, and most academic calendars use the US convention.
- Spreadsheets are split:
ISOWEEKNUMis ISO,WEEKNUMdefaults to US.
Check any date in seconds
Our Week Number Calculator shows both the ISO and US week for any date, plus the start and end of each week. Handy when you need to turn "W12" into actual delivery dates.