Week Number Calculator
ISO-8601, US, day-of-year
—
ISO-8601 week
—
ISO week (Mon-start)
—
US week (Sun-start)
—
Day of year (1-366)
—
Day of week
—
ISO year
—
Weeks in this ISO year
ISO-8601 is the one that matters
Most business systems, European payroll, and supply chains use ISO weeks. Week 1 is the week containing the year's first Thursday — equivalently, the week containing Jan 4. The US convention simply starts Week 1 on Jan 1 and counts Sundays.
Related tools
Why ISO weeks sometimes claim Jan-1 belongs to last year
- ISO Year 2026 starts on Mon Dec 29, 2025 (because Jan 1 2026 is a Thursday, week 1 starts the preceding Monday)
- Dec 29-31 2025 are officially \"Week 1 of 2026\" in ISO — it's normal
- The first 53-week years recur on specific patterns; 2026 is a 52-week year, while 2020 was 53 weeks
- Microsoft Excel's WEEKNUM with mode 21 returns ISO week; default mode uses the US convention
Payroll gotcha: if you're paid weekly, your December payslip might show \"Week 53\" in some tax years — a different withholding calculation applies in those years.