ISO Week Number Calculator Guide

ISO weeks start on Monday, and week 1 is defined by the first Thursday of the year. Dates near New Year can therefore belong to the previous or next ISO week-year.

How to use the ISO Week Number Calculator

1. Choose a calendar date.
2. Run the calculator to obtain the ISO week label and weekday.
3. Use the ISO week-year shown by the tool, not automatically the calendar year.

A practical example

January 1 can fall in week 52 or 53 of the previous ISO year. The calculator shifts to the week Thursday before deriving the year and number.

The ISO Week Number Calculator runs locally in the browser, so the input is available for the calculation without being uploaded by this tool.

Checks before you use the output

• ISO weekday numbers run Monday 1 through Sunday 7.
• Locale-specific week systems may start Sunday and produce different results.
• Store full dates when the exact day matters; a week label is less precise.

Use the result as part of a review

Find the ISO week-year, week number, weekday, and day of year. The output is designed to make a small task faster, but it should still be checked against the requirements of the destination system, document, or decision.

Privacy and safe sample data

Processing happens in the current browser tab. Even so, remove passwords, authorization values, customer records, and other confidential data before copying results into chat, tickets, or public examples.