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Week Number Calculator

ISO week ↔ calendar date.

ISO 8601
2024-W01-1
2024-W01
ISO year
2024
Week
1
Weekday
Monday(1)
Date
2024-01-01
Week range (Mon–Sun)
2024-01-01 → 2024-01-07

ISO 8601: weeks start on Monday. Week 1 contains the year’s first Thursday (the week of January 4). Late December / early January may fall in a different ISO year. Calendar dates only — timezone-independent.

Acerca de Week Number Calculator

ISO week numbers follow ISO 8601: weeks start on Monday, and week 1 contains the year’s first Thursday (the week of January 4). Late December and early January can fall in a different ISO year than the Gregorian calendar. toco.tools converts both ways—date → week and week → date—showing ISO year, week number, weekday (1=Mon … 7=Sun), a compact label (e.g. 2024-W01-1), and the Monday–Sunday range. Only calendar dates are used (timezone-independent). Everything runs in your browser.

Cómo usarla

  1. Choose Date → Week or Week → Date
  2. Enter a date, or ISO year, week, and weekday
  3. Review the ISO week label and week range

Preguntas frecuentes

What is an ISO week number?

An ISO 8601 week date. Weeks start on Monday; week 1 contains the year’s first Thursday.

Why can January 1 fall in the previous year’s week?

If Jan 1 is Fri–Sun, it belongs to the previous ISO year’s last week (often week 52 or 53).

Is my date sent anywhere?

No. All calculation runs in your browser.