ISO Week Numbers Explained: How Week 1 Is Decided

ISO 8601, the international standard for dates and times, defines weeks with three rules:

1. Weeks start on Monday. Monday is day 1 and Sunday is day 7.
2. Week 1 is the week that contains the year's first Thursday. Equivalently, it is the week containing 4 January, and the first week with at least four of its days in the new year.
3. Weeks are never split. Every week belongs entirely to one "week-numbering year", even when its days straddle 31 December.

Thursday is the tie-breaker because it is the middle of a Monday-to-Sunday week: whichever year holds the Thursday holds the majority of the week. The same logic means 28 December is always in the last week of its year, and 4 January always in week 1.

In ISO notation, Wednesday 23 September 2026 is 2026-W39-3: ISO year 2026, week 39, day 3. The week number tool shows the current week and looks up any date.

Because week 1 must begin on a Monday, it can start as early as 29 December or as late as 4 January.

ISO year	Week 1 begins	1 January falls on	Weeks in the year
2025	Mon 30 Dec 2024	Wednesday	52
2026	Mon 29 Dec 2025	Thursday	53
2027	Mon 4 Jan 2027	Friday	52
2028	Mon 3 Jan 2028	Saturday	52
2029	Mon 1 Jan 2029	Monday	52
2030	Mon 31 Dec 2029	Tuesday	52

Monday 29 to Wednesday 31 December 2025 are days of 2026-W01, not of 2025. At the other end, 2026 has a week 53 that runs from Monday 28 December 2026 to Sunday 3 January 2027, so Friday 1 January 2027 is 2026-W53-5. In 2029, calendar year and ISO year line up exactly, because 1 January is itself a Monday.

A normal year has 365 days, which is 52 weeks and one day; a leap year is 52 weeks and two days. Every five or six years the spare days add up to a whole extra week. An ISO year has 53 weeks if:

• 1 January is a Thursday, or
• it is a leap year and 1 January is a Wednesday.

In either case the year contains 53 Thursdays. The pattern repeats every 400 years, and 71 of those 400 years are long ones. Recent and upcoming examples are 2015, 2020 (a leap year starting on Wednesday), 2026, 2032 and 2037. Of 2025 to 2030, only 2026 qualifies: 1 January 2026 was a Thursday.

Week 53 causes real work: a business comparing weekly sales year on year has nothing to set it against, and budgets split into 52 weekly parts need a rule for it.

The United States has no national standard for week numbers, but the common convention, used by many American calendars and by Excel's default WEEKNUM function, is different on both points:

• Weeks start on Sunday.
• Week 1 is the week containing 1 January, however few days of it fall in the new year.

The result is partial weeks at both ends of the year, and up to 54 weeks. In 2028, a leap year starting on a Saturday, 1 January is alone in week 1 and Sunday 31 December is alone in week 54. The two systems also drift apart. In 2026 they agree from Monday to Saturday, but each Sunday already belongs to the next US week. In 2027 and 2028, from Monday to Saturday, the US number is one higher than the ISO number for almost the whole year: Wednesday 14 July 2027 is ISO week 28 but US week 29. Many countries in the Middle East start the week on Saturday, adding a third variant.

Software reflects the split. In Excel, use ISOWEEKNUM, or WEEKNUM with 21 as the second argument, for ISO weeks; Google Sheets also has ISOWEEKNUM. In C and Python date formatting, %V is the ISO week and must be paired with %G, the ISO year, rather than %Y. Java's "YYYY" pattern is also a week-based year, and using it by mistake instead of "yyyy" is a classic bug that prints the wrong year for a few days every late December. Timestamps stored as Unix time avoid the problem until the moment they are formatted.

• Everyday life in northern Europe. In Sweden, Norway, Denmark and Finland people routinely say "week 12" for holidays and deadlines, and wall calendars print the numbers. In Germany the Kalenderwoche, abbreviated KW, is standard in offices.
• Manufacturing and logistics. Production plans, container schedules and delivery promises are often quoted as a week rather than a day.
• Retail and finance. Weekly sales, staffing and stock reports are compared by week number from year to year.
• Software teams. Sprints and release trains are commonly named after the week they start in. The sprint timer tracks progress through one.

For international teams, write the full form, such as 2026-W39, so no one has to guess the year or the system.

You can calculate an ISO week with one formula. Take the ordinal date (day of the year, 1 to 365 or 366) and the ISO weekday (Monday = 1 to Sunday = 7):

week = (ordinal − weekday + 10) ÷ 7, rounded down

For Wednesday 23 September 2026, the ordinal is 266 and the weekday 3: (266 − 3 + 10) ÷ 7 = 39, so week 39. Two edge cases need a check. A result of 0 means the date belongs to the last week of the previous year: Friday 1 January 2027 gives 6 ÷ 7, which rounds down to 0, so it falls in 2026-W53. A result of 53 in a year that has only 52 weeks means week 1 of the next year: Monday 30 December 2024, ordinal 365 in a leap year, gives 53, but 2024 had 52 weeks, so it is 2025-W01.

For everyday use, let tools do it. The week number page shows the ISO week for any date, the date calculator adds or subtracts days (seven per week), and the current time page confirms today's date. To count down to the start of a particular week, point the countdown at its Monday.