What is an ISO week date?

An ISO week date (ISO 8601) represents a date as YYYY-Www-D — for example, 2025-W01-3 means the 3rd day (Wednesday) of week 1 of ISO year 2025. Week 1 is the week containing the first Thursday of the calendar year.

Why can the ISO year differ from the calendar year?

ISO week 1 is defined as the week with the year's first Thursday. So Jan 1–3 might belong to the last week (W52 or W53) of the previous ISO year, and Dec 29–31 might fall in week 1 of the next ISO year.

How many weeks does an ISO year have?

Most ISO years have 52 weeks. A year has 53 weeks if Jan 1 falls on Thursday, or if it's a leap year and Jan 1 falls on Wednesday or Thursday. About 71 out of 400 years have 53 weeks.

What day does ISO week start on?

ISO weeks always start on Monday (day 1) and end on Sunday (day 7). This differs from some locales where weeks start on Sunday.

Is W01 always the week containing January 1?

Not necessarily. W01 contains the first Thursday of the year, which means it is the week with at least 4 days in January. If Jan 1 is Friday, Saturday, or Sunday, it belongs to the last ISO week of the previous year.

ISO Week Date Converter

Name: ISO Week Date Converter
Availability: InStock
Author: Nham Vu

Convert any calendar date to ISO 8601 week date format (YYYY-Www-D) and back, with week number, day of week, and year boundary details.

Calendar Date → ISO Week

Enter a calendar date to get its ISO 8601 week date.

Pick a date above to convert.

ISO Week → Calendar Date

Enter an ISO week date (YYYY-Www-D) to find the calendar date.

Year

Week (1–53)

Day (1–7)

Day: 1=Mon, 2=Tue, 3=Wed, 4=Thu, 5=Fri, 6=Sat, 7=Sun

Enter year, week, and day above to convert.

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Summary

Convert any calendar date to ISO 8601 week date format (YYYY-Www-D) and back, with week number, day of week, and year boundary details.

How it works

ISO 8601 defines a week date format as YYYY-Www-D, where W indicates week, ww is the 2-digit week number (01–53), and D is the day of week (1=Monday through 7=Sunday).
The ISO week year may differ from the calendar year: week 1 is the week containing the year's first Thursday, so Jan 1–3 can belong to the prior year's last week.
Enter a calendar date in the left panel to see its ISO week date representation, or enter an ISO week date on the right to find the corresponding calendar date.
The tool validates all inputs and shows which calendar year the ISO week belongs to, the first and last day of that week, and the day name.

Use cases

Verify ISO week numbers for payroll and financial reporting periods.
Convert project deadlines between calendar dates and ISO week identifiers.
Check whether Jan 1 falls in week 1 or the last week of the prior year.
Find the Monday start and Sunday end of any ISO week for scheduling.
Validate ISO 8601 week date strings in data pipelines or APIs.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu