What Is an ISO Week Number?

A week number in one sentence

An ISO week number says which calendar week of the year a date is in, using the rules of the international standard ISO 8601. The rules are short. A week starts on Monday and ends on Sunday, and week 1 of a year is the week that contains the first Thursday of that year. Everything else, from the odd behaviour around New Year to the existence of week 53, follows from those two sentences.

Take today as an example. Thursday 22 October 2026 is in ISO week 43. That week runs from Monday 19 October to Sunday 25 October 2026, so Friday 23 October is in week 43 as well. When someone says "see you in week 43", they mean one of those seven days, and the calculator on this page turns the number into the dates in one step.

The two rules

The first rule fixes the weekdays: Monday is the first day of the week and Sunday the last. That is different from calendars in the United States, which usually start the week on Sunday, but it matches how most of Europe and many businesses think about a working week.

The second rule fixes which week is week 1. It is the week that contains the first Thursday of the new year. There are two other ways to say the same thing that are easier to apply. Week 1 is the week that contains 4 January. It is also the first week that has at least four of its seven days in the new year. All three statements describe the same week every time.

Why Thursday? A week runs Monday to Sunday, so if Thursday falls in January, at least four days of that week, Monday to Thursday, are in the new year. The week is then more January than December, and it is fair to give it to the new year. If Thursday still falls in December, the week belongs to the old year.

Reading a number such as week 43 of 2026

A full ISO week reference has two parts, the week-year and the week number. For most dates the week-year is the same as the calendar year, but near New Year it can differ. The standard writes it as 2026-W43, and with a weekday as 2026-W43-4, where 4 stands for Thursday, the fourth day of the week counted from Monday.

The week-year is the year that contains the Thursday of the week. This is why Friday 1 January 2027 is in week 53 of 2026: the Thursday of that week is 31 December 2026. The following Monday, 4 January 2027, starts week 1 of 2027.

The same logic works in the other direction. Monday 29 December 2025 is already in week 1 of 2026, because the Thursday of that week is 1 January 2026. A person who looks at a calendar and sees 29 December in "week 1" is not looking at an error.

How many weeks does a year have?

A common year has 365 days, which is 52 weeks and one day. A leap year has 366 days, which is 52 weeks and two days. Because of the extra days, the ISO weeks do not line up with the calendar year, and every year has either 52 or 53 full ISO weeks.

A year has 53 weeks if 1 January is a Thursday, or if it is a leap year and 1 January is a Wednesday. In both cases the year has a spare Thursday that completes an extra week. The years 2015, 2020 and 2026 have 53 weeks. The years 2027, 2028 and 2029 have 52 weeks. Over a full 400-year cycle of the Gregorian calendar there are 71 years with 53 weeks, about 18 percent.

The result is that week 53 exists only some of the time. Software that expects exactly 52 weeks in every year will fail on week 53, which is one of the most frequent bugs in date handling. The third tab of the calculator shows the number of weeks for any year from 1 to 9999.

Week numbers in daily life

In Germany, Austria, Switzerland, the Netherlands, the Nordic countries and many other places, people talk about the calendar week as a normal part of planning. A project plan might say "release in week 47", a supplier might promise delivery "in KW 12", and a school might publish its holidays by week number. In English-speaking countries the habit is less common in private life, but it is standard in logistics, manufacturing, retail and software teams.

The reason is practical. A week number is short, it is unambiguous once both sides use the ISO rule, and it avoids picking a weekday. Saying "week 47" leaves room to deliver on any day of that week, and everyone can see that it means mid-November in 2026, from Monday 16 November to Sunday 22 November.

The number carries one risk. If a person uses a different rule, for example the US rule with weeks starting on Sunday, the same date can get a different number. When a contract or schedule depends on a week number, it is worth writing the rule next to it or using the ISO form such as 2026-W47.

How this calculator uses ISO weeks

The first tab takes any date and shows its ISO week number as the large result. The sentence under it names the Monday and the Sunday of that week, and the rows below show the week-year, the weekday, the day of the year and the number of weeks in that ISO year. For comparison there are two extra rows with the US-style number and with the Monday-start number where week 1 contains 1 January.

The second tab works backwards. Enter a year and a week number, and the calculator lists the seven dates from Monday to Sunday. If you enter week 53 for a year that has only 52, it says so instead of inventing a date.

All of it runs in your browser. Dates are turned into whole days in UTC, so your time zone and daylight saving time cannot move a date to a different week. The answer for a given date is the same in every country.