ISO week number
Find an ISO week number, week-year and Monday–Sunday dates for a calendar date.
- 1Add input
- 2Adjust settings
- 3Get your result
Tool input and files are processed in this browser without being uploaded.
Before you start
Enter a calendar date to find its ISO week code, week-year and Monday–Sunday range. ISO weeks start on Monday, and week 1 contains the year’s first Thursday. Dates near New Year can belong to a different week-year.
How to use this tool
- Choose a date or use Today or the New Year example.
- Calculate the ISO week, then check the week-year and Monday–Sunday range, especially near New Year.
- Copy the ISO week code or number, or download the JSON result.
Supported inputs and limits
Choose a calendar date with the browser control. Its display format may vary, but the value is stored as YYYY-MM-DD without a time or time zone.
This follows ISO 8601. Some US calendars use Sunday-start weeks and number them differently.
The calculation runs in your browser; no account or upload is needed.
Worked example
Example input
2021-01-01
Example options
{}Example output
{
"date": "2021-01-01",
"year": 2020,
"week": 53,
"isoWeek": "2020-W53",
"weekday": 5,
"weekStart": "2020-12-28",
"weekEnd": "2021-01-03"
}When something does not work
Choose a real calendar date. Times, offsets and year 0000 are not accepted.
Frequently asked questions
Why does January 1 sometimes show the previous year?
ISO week 1 is the week containing the first Thursday. January 1 can therefore belong to the previous ISO week-year; use the year in the ISO week code.
Does this match US calendar week numbers?
Not always. This tool uses ISO 8601: Monday starts each week and week 1 contains the first Thursday. Sunday-start calendars may return a different week number.
Can I enter a time or time zone?
No. Choose a calendar date. The result describes that day, not a timestamp converted between time zones.
Documentation & further reading
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