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W##

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.

About 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.

How to use

  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

FAQ

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.