Week Number Lookup

The ISO week for any date

Choose a date to see its ISO week number, week-numbering year, standard week label, and the Monday-to-Sunday range containing it. The calculation is performed locally in your browser.

A result such as 2026-W05 means week 5 of ISO week year 2026. The two-digit week form is useful in schedules, reports, filenames and data exchanged between systems.

How ISO weeks work

ISO 8601 weeks begin on Monday and end on Sunday. Week 1 is the week containing January 4. The same rule can be stated another way: week 1 contains the first Thursday of the year.

That Thursday rule makes every numbered week contain most of its days in the year named by the week. It also means January 1 is not automatically in week 1. If New Year’s Day falls late in the week, it belongs to week 52 or 53 of the preceding week-numbering year.

The week year can be surprising

A calendar date has an ordinary year and, under ISO 8601, a week-numbering year. They agree for almost the whole year but can differ near New Year. For example, 29 December can already be in week 1 of the following week year, while 1 January can remain in the final week of the previous one.

This is why the lookup shows the week year and an ISO label instead of displaying a bare number. “Week 1” alone is not enough to identify a week, and pairing it automatically with the date’s calendar year can produce a plausible but incorrect result.

Why some years have week 53

A common year has 365 days: 52 complete weeks plus one day. A leap year has two extra days beyond 52 weeks. As those spare days accumulate against the Monday boundary, some ISO week years receive a complete 53rd week.

In practical terms, an ISO year has week 53 when its dates and weekdays align to require one. The tool calculates the boundary from the selected date rather than relying on a fixed list.

Other week-numbering systems

Not every calendar uses ISO weeks. Some start the week on Sunday, consider the week containing January 1 to be week 1, or use locale-specific “first week” rules. Those systems can give a different number for the same date.

This lookup always uses ISO 8601 and names that convention beside the result. It treats the input as a calendar date in UTC, which prevents daylight-saving transitions from shifting the answer by a day.

Frequently asked questions

What week number is it?

Choose today's date to see the current ISO week. The result includes both the week number and its week-numbering year, plus the Monday and Sunday that bound it.

What is an ISO week number?

ISO 8601 numbers weeks from Monday through Sunday. Week 1 is the week containing January 4, which is also the week containing the year's first Thursday.

Why can the week year differ from the calendar year?

The first and last few days of a calendar year may belong to a week centered in the neighboring year. For example, a January date can still be in week 52 or 53 of the previous ISO week year.

Can a year have 53 ISO weeks?

Yes. Most ISO week years have 52 weeks, but some have 53. This occurs when the calendar alignment leaves enough days for a complete numbered week at the end.

Does every country number weeks this way?

No. Some calendars start weeks on Sunday or define week 1 differently. This tool uses the ISO 8601 convention so the answer is explicit and repeatable.

What does a label such as 2026-W05 mean?

It is the ISO week-date form: week 05 in week-numbering year 2026. Including the year avoids ambiguity around New Year.

Does the lookup use my time zone?

No time zone is needed for a plain calendar date. The arithmetic uses UTC internally so daylight-saving changes cannot move a date into a neighboring week.