Week Number Lookup

How it works

This lookup uses the ISO-8601 week numbering rule, the one used by business calendars across Europe and by most planning software: a week always runs from Monday to Sunday, and week 1 of a year is the week that contains that year's first Thursday (equivalently, the week containing 4 January). Because of that rule the first days of January can still belong to week 52 or 53 of the previous year, and the last days of December can already belong to week 1 of the next one — which is why the tool shows the ISO week-year separately from the calendar year. A year has 53 ISO weeks when 1 January falls on a Thursday, or when it is a leap year starting on a Wednesday; otherwise it has 52.

Enter any date to see its week number, the Monday and Sunday that bracket it, the ISO week date (for example 2026-W32-7), the day of the year and how many weeks the ISO year has. The US/Canada week number is listed too, since North American calendars start the week on Sunday and count the week containing 1 January as week 1 — the two systems often disagree by one. You can also work backwards from a week number to its date range, or list every week of a year and download it as a CSV. All the arithmetic runs in UTC in your browser with plain JavaScript, so daylight-saving jumps can never shift a result, nothing is uploaded, and there are no ads or sign-ups.