หมายเลขสัปดาห์ (ISO 8601)

The ISO 8601 week number is a standardized way to number the weeks of a year (1-52 or 1-53), where a week starts on Monday and the year's first week is the one containing the first Thursday of January. This differs from a casual "since the start of the year" count.

How to use it

• Date → week number: enter a date to find its ISO 8601 week number and that week's ISO year.
• Week number → date: enter a year and week number to get that week's start (Monday) and end (Sunday) dates.
• For years with 53 weeks (rarer than 52), the tool handles that edge case correctly too.

Common uses

• Planning sprints or weekly reporting in project management, where ISO numbering is the convention.
• Cross-checking a week number against the ISO standard when it differs from what a calendar or another system shows.
• Finding the exact start and end dates of a specific work week for a schedule or invoice.

Things to keep in mind

A week's ISO year doesn't always match the calendar year: the first days of January can belong to the last week of the previous ISO year, and the last days of December to the first week of the next.

The number of weeks in an ISO year is 52 or 53, depending on what day of the week January 1 falls on and whether the year is a leap year.