ISO Week from Date (Complete)

Production planning systems, construction schedules and management reports all talk in week 32, which means turning an ordinary date into an ISO 8601 week. Counting on a calendar goes wrong quickly, because the ISO rule is not divide by seven: week 1 is the week containing the first Thursday of the year. Enter year, month and day and the page returns the number.

The algorithm is the classic one: take the date, step to the Thursday of that week, and count the days since 1 January of that Thursday's year. That is why 1 January 2027, a Friday, comes out as week 53, since its Thursday is 31/12/2026 and the week belongs to ISO year 2026. Two things worth knowing: the result prints with four decimal places, as 19.0000, and the ISO year itself is never shown.

Whenever the date sits near a year boundary, record the ISO year next to the number, because week 53 on its own identifies nothing. The tool name and description also promise the weekday, yet the page outputs only the week number, so that part simply is not there. Feed it whole numbers: the fields accept decimals and out-of-range values, and the date slides somewhere else without warning when you do. The calculation runs in the browser, in UTC.