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ISO 8601 vs. US Week Numbering
Week numbers look simple until two calendars disagree. The Week Number Calculator shows both common conventions side-by-side so you can pick the right one for your purpose.
ISO 8601 (Monday-start)
- Weeks begin on Monday.
- Week 1 of a year is the week containing the first Thursday of that year (equivalently: the week containing January 4th).
- Week number format:
2026-W15. - Used in Europe, by most ERP/HR systems, and by the
datecommand on Linux (date +%V).
US (Sunday-start)
- Weeks begin on Sunday.
- Week 1 is the week containing January 1, so Dec 31 can fall in week 53 even when ISO puts it in week 1 of the next year.
- Used by US payroll, most American calendars, and Excel's default
WEEKNUM.
What Else You Get
- Day of year — the ordinal day (Jan 1 = 1, Dec 31 = 365/366).
- Days remaining — how many days are left in the calendar year, useful for planning and deadlines.
- Week range — the Monday–Sunday (ISO) and Sunday–Saturday (US) dates bounding the week.
Common Uses
- Filing bug reports that reference sprint or release weeks.
- Answering "what week is Labour Day?" style questions.
- Reconciling calendars between European and US teams.
- Producing consistent week IDs for reporting.
Privacy
All calculation happens in your browser. No dates leave your device.