About WeekNumber
WeekNumber answers one question two ways: what week is it? — in the international ISO 8601 system, and in the Sunday-start system common on US calendars.
Method
ISO 8601. Weeks run Monday to Sunday. Week 1 of an ISO year is the week containing the year’s first Thursday (equivalently, the week containing January 4). A date’s ISO year is the year of its week’s Thursday, so dates near a year boundary can belong to the neighbouring ISO year. An ISO year has 52 or 53 weeks; December 28 always falls in its last week, which is how the year calendars on this site are checked.
US convention. There is no single official US standard for week numbering. This site uses — and states wherever the number appears — the common calendar convention: weeks run Sunday to Saturday, and week 1 is the week containing January 1, whatever weekday January 1 falls on. Some American software counts differently (for example, numbering the first full week as week 1); when a number matters, compare the date ranges this site prints behind every result, not just the number.
All calculations run in your browser, in UTC, on calendar days only. Time zones and daylight saving can move a clock time, but they can never move a date’s week number once the date itself is fixed. Nothing you enter is sent anywhere or stored.
Sources & boundaries
The ISO rules follow ISO 8601 week dating, and agree with Excel’s
ISOWEEKNUM and Python’s isocalendar(). The 53-week rule (a year is
long when it starts on a Thursday, or a leap year starts on a Wednesday) is a direct
consequence of those definitions, and every year table on this site is generated from the same
definitions the calculator uses — nothing is typed in by hand. Week numbers are conventions,
not measurements: if a contract or employer defines weeks differently, their definition wins
for that purpose.
Who runs this
WeekNumber is part of a small family of free, single-purpose tools built by Jimmy | ORBITFRAME. The tools share one idea: the answer, immediately, with the working shown — and everything computed on your own device.