What week of the year is it? How ISO week numbers work

The ISO rule in three lines

The international standard ISO 8601 defines weeks like this: they start on Monday, week 1 is the one containing the first Thursday of the year (which is the same as the one containing 4 January) and a year has 52 or 53 weeks. The Python and PostgreSQL documentation both describe it exactly that way, and Excel has a dedicated function for it.

The practical consequence is that week 1 can start in December, and week 52 or 53 can end in January. The week number and the calendar year do not always travel together.

A full example: 2026

1 January 2026 is a Thursday. Since week 1 is the one containing that first Thursday, it starts on Monday 29 December 2025: those three December days already belong to week 1 of 2026. From there the weeks add up seven days at a time until, on 28 December 2026, a week 53 begins and ends on Sunday 3 January 2027. That is why 1 January 2027 (a Friday) is “week 53 of 2026”, and week 1 of 2027 does not start until Monday 4 January.

Why some years have 53 weeks

52 weeks is 364 days, and a year has 365 (or 366). The leftover days pile up until, every five or six years, they make up an extra week. The exact rule: a year has 53 ISO weeks when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday. The most recent 53-week years were 2015 and 2020; the next are 2026, 2032, 2037 and 2043. If your planning assumes “52 weeks a year”, 2026 will throw you off by one.

How to work out the week of a date by hand

It comes down to a single number: the days elapsed since the Monday of week 1.

1. Find the Monday of week 1 of the year: the Monday of the week that contains 4 January. In 2026, 29 December 2025.
2. Count the days from that Monday to your date. From 29 December 2025 to 3 October 2026 is 278 days.
3. Divide by 7, keep the whole part and add 1: 278 ÷ 7 = 39.7 → 39 + 1 = week 40.

And the other way round, to find which days a week covers: the Monday of week N is the Monday of week 1 plus 7 × (N − 1) days. Week 47 of 2026 starts 322 days after 29 December 2025: Monday 16 November, ending on Sunday 22.

Why Excel (and others) sometimes gives a different number

Excel's WEEKNUM function uses a different system by default: week 1 is the week containing 1 January, and the week starts on Sunday. Microsoft documents it that way, calling this “System 1” and the ISO 8601 method “System 2”. With the two rules, the same date can get different numbers: Sunday 4 January 2026 is week 1 in ISO (it closes the week that began on 29 December) and week 2 in Excel's default system. And 1 January 2027, which ISO counts as week 53, is week 1 in Excel's default system.

If you want the ISO number in Excel, you have two options: the ISOWEEKNUM function, available in Excel 2016 and later, or WEEKNUM(date, 21), where the second argument 21 selects the ISO system. If you share spreadsheets with someone in another country, that avoids arguments.

The most common mistake: mixing the calendar year with the ISO week

When you label “2026-W01” or “week 1 of 2026” you have to use the ISO year, not the calendar year. The typical case: Monday 30 December 2024 is week 1 of 2025. If a program formats the calendar year and the ISO week separately, it will write “2024-01”, which is the wrong date and also sorts incorrectly in any list. In Python, for example, the right format uses %G (ISO year) with %V (ISO week): %G-W%V-%u gives “2025-W01-1” for that day. PostgreSQL recommends using the isoyear field together with week for the same reason.

The full ISO 8601 notation is year, W, week and weekday: 2026-W53-7 is Sunday 3 January 2027.

When to check which week is meant

ISO is the common language, but some calendars do not follow it: some companies number their own fiscal or production weeks, and in the United States weeks have traditionally run Sunday to Saturday. If someone tells you “delivery in week 12”, the safe thing is to confirm the dates: in ISO 2026, 16 to 22 March.

Skipping the arithmetic

The week number calculator gives you the ISO number of any date, the Monday and Sunday of that week, the day of the year, the quarter and, for comparison, the number under the US system. It also works in reverse: enter a year and a week and it returns the dates. If what you need is the days between two dates or to add weeks to a deadline, use the date difference tool or add days to a date, and for deadlines that count working days only, the business days calculator.

For how dates are stored in systems, see the Unix timestamp guide, and for how daylight saving shifts them, when the clocks change.

Sources and further reading

• Python: datetime, date.isocalendar() and the %G, %V, %u directives (ISO 8601)
• PostgreSQL: date/time functions, week and isoyear fields (week 1 contains 4 January)
• Microsoft: WEEKNUM function (System 1, System 2 ISO 8601 and return_type 21)
• Microsoft: ISOWEEKNUM function

Figures checked on 3 October 2026.