Find the ISO 8601 week number of any date — and the exact dates of any week number.
Week numbers are the working calendar of factories, logistics, retail planning and engineering projects: "we ship in week 42" is unambiguous across countries and languages in a way that a date format never is. The standard behind it is ISO 8601, and its rules are short but strict — a week always starts on Monday and ends on Sunday, and week 1 of a year is the week that contains the first Thursday of January.
That Thursday rule is what makes the edges surprising. Because a week belongs to the year where most of its days fall, the first days of January can belong to the last week of the previous year, and the last days of December can already belong to week 1 of the next. 1 January 2027, for example, is a Friday and sits in week 53 of 2026. For the same reason a year has 52 or 53 ISO weeks, never 52 and a half: 53-week years happen when the year starts on a Thursday, or on a Wednesday in a leap year.
This calculator implements the rule directly rather than approximating: it moves from your date to the Thursday of its week, which determines the ISO week-year, and counts weeks from the Thursday of week 1. The reverse mode does the same in the opposite direction and refuses week 53 in years that do not have one. Both directions are calculated in your browser, with no network request involved.
ISO week: move the date to the Thursday of its week (Monday = day 0, so Thursday = date + 3 − weekday index); the year of that Thursday is the ISO week-year; the week number is the number of whole weeks between the Thursday of week 1 — the Thursday of the week containing 4 January — and that Thursday, plus 1. Reverse: Monday of week W = Thursday of week 1 − 3 + (W − 1) × 7. A year has 53 ISO weeks when 28 December falls in week 53.
Press "Today" and the tool shows the ISO week you are in, along with its Monday-to-Sunday dates. The result follows ISO 8601, the same standard used by planning systems in Europe and by most logistics operations.
Because ISO weeks belong to the year of their Thursday, and a week is never split between two years. If 1 January falls on a Friday, Saturday or Sunday, that whole week still belongs to the previous year.
Yes. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday if it is a leap year. It happens roughly every five to six years — 2020, 2026 and 2032 are examples.
On Monday. ISO 8601 fixes Monday as the first day of the week, which is why the numbering here can differ from calendars in the United States, where weeks are often counted from Sunday.
No. Both directions of the calculation run in your browser and nothing is transmitted or stored.
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