Certified tool
Week Number Calculator
ISO-8601 week number and year for a calendar date.
ISO-8601 week
2026-W01
Thursday, January 1, 2026 (2026-01-01) falls in ISO week 1 of 2026.
ISO weekday: 4 (Monday = 1).
US-style Sunday-start week number in 2026, where week 1 contains January 1: 1. This is secondary information, not the primary ISO result.
Assumptions
- Primary result is ISO-8601: weeks start Monday, and week 1 contains January 4.
- Early January dates may belong to the previous ISO week-year.
- A secondary US-style Sunday-start week number is shown for comparison only.
- This is not a fiscal-week or school-week calculator.