ISO-8601 Week Numbers & Ranges

What Week of the Year Is It (ISO-8601)

Q: Can January dates belong to the previous year's week number?

Yes. If January 1st falls on a Friday, Saturday, or Sunday, those days have fewer than 4 days in the new year, so they belong to Week 52 or 53 of the preceding year.

Q: What is the format for an ISO-8601 week string?

The official ISO format is YYYY-Www (e.g. 2026-W39), and YYYY-Www-D for a specific weekday (e.g. 2026-W39-7 for Sunday).

Check the current week number according to the international ISO-8601 standard, including exact Monday start and Sunday end boundary dates.

ISO-8601 Standard

International Week Number Tracker

Check Date:

ISO Year

Week --

--- – ---

-- days left in this ISO week

Under international standard ISO-8601, weeks begin on Monday and end on Sunday. Week 1 is the first week containing at least 4 days of the new calendar year (the week with the year's first Thursday).

ISO-8601 Week Numbering Rules

The International Organization for Standardization (ISO) established standard 8601 to eliminate ambiguity in global business and software:

Monday is Day 1: The business week always starts on Monday and concludes on Sunday (Day 7).
First Thursday Rule: Week 1 is defined as the week with the year's first Thursday (or containing January 4th).
52 or 53 Weeks: A calendar year typically has 52 weeks, but years starting on a Thursday (or leap years starting Wednesday) contain 53 weeks.

Common Questions

Frequently Asked Questions

Can January dates belong to the previous year's week number? +

What is the format for an ISO-8601 week string? +

What Week of the Year Is It (ISO-8601)

International Week Number Tracker

ISO-8601 Week Numbering Rules

Frequently Asked Questions

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