Week Number Calculator

How to use the week number calculator

1. When the page loads it shows the week number for today, so you can answer "what week is it?" at a glance.
2. Pick any date to see its week number instead — useful for planning, payroll periods, or referring to an ISO week like 2026-W30.
3. Compare the ISO 8601 and US figures side by side. They often differ; the section below explains why.
4. Press Jump to today at any time to return to the current week.

ISO 8601 week numbers vs US week numbers

There is no single "week number" — it depends on two choices: which day the week starts, and which week gets to be number 1.

ISO 8601 is the international standard used across Europe, in business and manufacturing, and by most software. Its rules are strict:

• Weeks start on Monday.
• Week 1 is the week containing the year's first Thursday — equivalently, the week that contains January 4.
• A consequence is that the first days of January can belong to the previous ISO year. January 1, 2021 sits in 2020-W53, and late December can spill into week 1 of the next year.

The common US convention is looser and appears in many American calendars and spreadsheets:

• Weeks start on Sunday.
• Week 1 is simply the week containing January 1, so January 1 is always in week 1 and the first week is usually a partial week.

Because US week 1 starts earlier and weeks begin a day sooner, the US week number is frequently one higher than the ISO number for the same date. Around New Year the two can disagree by a full year of labelling. Our ISO vs US week numbers guide walks through a worked example across the year boundary.

All of this is computed from local date components, so the week number never flips because of a timezone conversion. If you need to state a date unambiguously in the ISO system, use the YYYY-Www form shown above (for example, 2026-W30).