How ISO weeks work

One rule explains almost everything about ISO week numbers: a week belongs to the year that contains its Thursday.

ISO 8601, the international standard for dates, also defines a complete week calendar. Its weeks always run Monday to Sunday, and every week is numbered within an “ISO year” that is close to — but not exactly — the calendar year. Three definitions do all the work:

• Weeks start on Monday and end on Sunday.
• Week 1 of an ISO year is the week containing the year’s first Thursday. The same week can be picked out another way: it is the week containing January 4, and it is the first week that has four or more days in the new year. All three descriptions always name the same week.
• A date’s ISO year is the year of its week’s Thursday. That is the rule that pushes some late-December days into next year’s week 1 and some early-January days back into last year’s week 52 or 53.

Why some years have 53 weeks

A year has 52 full weeks plus one spare day (two in a leap year). Whether those spare days add up to a whole extra numbered week depends on which weekday the year starts on. An ISO year gets 53 weeks exactly when it begins on a Thursday — or, in a leap year, when it begins on a Wednesday. Equivalently: a year has 53 ISO weeks if and only if it ends on a Thursday, or on a Friday in a leap year.

2026 is a 53-week year. It begins on Thursday, January 1, 2026, so its weeks run all the way to week 53, which ends on Sunday, January 3, 2027. Long years are not rare — 71 of every 400 years have 53 ISO weeks, roughly one year in six. The full list is on the 53-week years page, and every week of 2026 is dated in the 2026 calendar.

Finding a week number by hand

The reliable hand method is the “nearest Thursday” trick:

1. Take the date and find the Thursday of its week (if the date is a Monday, its Thursday is three days ahead; if it is a Sunday, its Thursday was three days back).
2. The ISO year is simply the year that Thursday falls in.
3. Count how many Thursdays have passed since the first Thursday of that year, add one — that is the week number. In practice: divide the Thursday’s day-of-year by 7 and round up.

Worked example: January 1, 2023 was a Sunday. The Thursday of its week was December 29, 2022 — in the old year — so the date belongs to ISO year 2022. December 29 was the 52nd Thursday-bound week of 2022, making the date ISO 2022-W52, even though the calendar already said 2023. The calculator does this instantly for any date and shows the US count beside it.

Writing ISO week dates

The standard also defines a written form: 2026-W44-2 means ISO year 2026, week 44, day 2 (Tuesday) — in ordinary dates, October 27, 2026. You will meet this format in European timetables, freight and production planning, and anywhere a schedule is published as “KW” (Kalenderwoche) in German-speaking countries.

Common questions

Is week 1 always the week of January 1?

No — that is the US convention. In ISO counting, if January 1 falls on a Friday, Saturday or Sunday, it still belongs to the last week of the previous ISO year, because that week’s Thursday is in December. Week 1 starts on the Monday of the week containing January 4.

Does every country use ISO weeks?

No. ISO weeks are standard across most of Europe and in international business, but the US, Canada and several other countries traditionally start weeks on Sunday and count week 1 from January 1. The differences are laid out in US vs ISO week numbers.

What is the week number in Excel or code?

Excel’s ISOWEEKNUM(date) and Python’s date.isocalendar().week both return the ISO number, matching this site. Plain WEEKNUM in Excel defaults to the US-style system, which is a frequent source of off-by-one-week surprises in shared spreadsheets.