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Week Number

See the current week number, or the week any date falls in, under both rules that are actually in use: ISO 8601, where weeks start on Monday and week 1 holds 4 January, and the US convention, where weeks start on Sunday and week 1 holds 1 January. Both numbers are on the page at once, alongside the exact days the week covers, the day of the year, and a table of every week of the year with its start and end dates.

ISO week numberReadyPick a date to see the week it falls in.
Date
Convention

ISO 8601: weeks run Monday to Sunday, and week 1 is the week holding 4 January. Standard across Europe and in most software. A week belongs to the year that holds its Thursday, so the first days of January can fall in the last week of the year before.

Two conventions, both shown. ISO 8601 is the international standard and the one meant by "week 36" in most of Europe and in most software. The US convention is what a spreadsheet returns from WEEKNUM unless you ask it for the ISO rule. How far apart they run depends on the weekday 1 January falls on: in a year opening Sunday through Thursday they agree every day except Sundays, when the US number is one higher, and in a year opening on a Friday or a Saturday the US number is one higher all week and two higher on Sundays. Neither is wrong, so this page prints both rather than picking for you. Exact by the ISO 8601 rule: weeks start on Monday and week 1 is the week with 4 January. The US convention (weeks start Sunday, week 1 holds 1 January) is offered separately, and the two can differ by one or two, or fall in different years, so both numbers are always shown.

Common questions

What week number is it right now?
The big number at the top is the week holding today. Today is read from your device's own calendar in the browser, so it is the date on your wall rather than the UTC date, which is already a different day for part of the world. Pick any other date in the field to see its week instead, and the Today button brings it back.
What is the difference between the ISO and US week numbers?
Two things: which day starts the week, and which week gets number 1. ISO 8601 weeks run Monday to Sunday and week 1 is the week containing 4 January, which is the same as saying the week containing the year's first Thursday. The US convention runs Sunday to Saturday and week 1 is the week containing 1 January, however few days of it fall in the new year. How far apart the two numbers run depends on the weekday 1 January falls on. In a year opening Sunday through Thursday, which includes 2026, they agree on every day except Sundays, where the US number is one higher because a US week has just begun while the ISO week is still finishing. In a year opening on a Friday or a Saturday the US number is one higher all week and two higher on Sundays, because the stub of days before that year's first Sunday takes a whole number to itself. Both are shown here so you never have to work out which one was meant.
Why do some years have 53 weeks?
A year is 52 weeks plus one day, or plus two days in a leap year, so the extra days accumulate and every five or six years the ISO calendar needs a 53rd week to absorb them. Precisely: an ISO year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 has 53; 2025 has 52. The US convention can go further still and produce a 54th week, but only in a leap year that opens on a Saturday, and that week is always exactly one day long: 31 December. 2000 was one, and 2028 is the next.
Which week are the first days of January in?
Under ISO 8601 they often belong to the year before. A week belongs to the year that holds its Thursday, so 1 January 2027, a Friday, is in week 53 of 2026, and 30 December 2024, a Monday, is already in week 1 of 2025. This is not an edge case that only a standard cares about: it is why an ISO week number always comes with its own year, written 2026-W36. The US convention never does this, because it restarts numbering on 1 January and keeps the calendar year, at the cost of first and last weeks that are usually cut short.
Why does Excel give a different week number?
Because WEEKNUM defaults to the US convention. WEEKNUM(date) and WEEKNUM(date, 1) start weeks on Sunday and put 1 January in week 1, which is the US column here. For the ISO number use ISOWEEKNUM(date), or WEEKNUM(date, 21). Google Sheets follows the same rules. If a spreadsheet and a European colleague disagree about which week a date is in, this is almost always why.
Do time zones or daylight saving change the answer?
No. Every calculation here is integer arithmetic on whole days, so a clock change can never add or remove one, and a date is treated as a calendar date rather than an instant. The only place local time is used is reading today off your device when the page opens, which is exactly where you want it: converting that through UTC is the bug that shows yesterday's week number to everyone west of Greenwich for part of every day.

Exact by the ISO 8601 rule: weeks start on Monday and week 1 is the week with 4 January. The US convention (weeks start Sunday, week 1 holds 1 January) is offered separately, and the two can differ by one or two, or fall in different years, so both numbers are always shown.