Week number
What week of the year is it? Here's today's ISO 8601 week number, or the week number of any date you want to check.
The ISO 8601 rule: Thursday decides
Under the ISO 8601 standard, weeks start on Monday and week 1 is the week that contains the first Thursday of the year (equivalently, the week that contains January 4). That's why January 1 or 2 can still belong to the last week of the previous year, week 52 or 53, and late December can already count as "week 1" of the new year. Each week belongs to whichever year its Thursday falls in. Note that this differs from the US convention used by some calendars, where weeks start on Sunday and week 1 simply contains January 1.
52 or 53 weeks? And what week numbers are for
Almost every year has 52 ISO weeks; a year has 53 when it starts on a Thursday, or when it's a leap year starting on a Wednesday (it happened in 2020 and happens again in 2026 and 2032). Week numbers are everywhere in business, the "CW" (calendar week) you see in supplier emails, shipping schedules, and manufacturing plans, and they're also used in schools and hospitals to organize rotations and shifts.
Where week numbers really matter
Outside offices almost nobody uses week numbers, but in industry, logistics and northern Europe they are the date: deliveries are scheduled «in week 34», production shifts are planned by week, and German and Scandinavian wall calendars have the numbers printed beside the days.
It is also the standard way of comparing periods across different years: months have different lengths and fall on different days, ISO weeks do not. For a series of sales or visits, week 12 of 2025 and week 12 of 2026 contain the same number of Saturdays and Sundays.
Week 1 and the year-end trap
The ISO rule says week 1 is the one containing the first Thursday of the year, and that produces results that look wrong: 1 January can sit in week 52 or 53 of the previous year, and 31 December can already sit in week 1 of the next one.
It is not a flaw, it is the consequence of wanting whole weeks, never split between two years. The equivalent and easier rule to remember is that week 1 is the one containing at least four days of the new year.
The American system is a different one
In the United States the week starts on Sunday and week 1 is simply the one containing 1 January, with no Thursday rule. The same day can therefore carry two different numbers, and the gap is one unit for much of the year.
It is the classic silent spreadsheet error: the same formula gives different results depending on the function used and the locale settings. Whenever a week number travels between two systems, the convention in use has to be stated.
Nearby tools
The weekday of a date comes from What day was it, and the days between two dates from Difference between dates. To count only usable days there is Working days between dates.