Week Numbers Around the World

Week numbers are a regional habit

The seven-day week is shared across almost the whole world, but the habit of numbering the weeks of the year is not. In Germany, Austria and Switzerland, "Kalenderwoche" or KW is part of the vocabulary of work. In Sweden and the other Nordic countries, calendars print the week number, and people say "vecka 43" in ordinary conversation. In the Netherlands, "weeknummer" is common in planning and in school holidays.

In the United States, the United Kingdom and many other English-speaking countries, week numbers are rare in private life, though many businesses use them for planning. The same applies to most of Asia and Latin America: week numbers exist in logistics, manufacturing and software, but not in the everyday talk of most households.

The first day of the week

Countries disagree on the first day. In most of Europe the week starts on Monday, and that is also the ISO standard. In the United States, Canada, Japan, Brazil and Israel, calendars commonly start the week on Sunday. In some countries, such as Iran, Afghanistan and Egypt, the week starts on Saturday.

Software uses the locale to decide. The Unicode locale data, which many operating systems and browsers use, records a first day and a minimum number of days in the first week for each region. If you change the region of a computer, the week numbers in its calendar may change with it.

The week-one rule in locale data

Besides the first day, a calendar needs a rule for week 1. In the locale data, this is expressed as the minimum number of days in the first week. A value of 4 matches the ISO rule that week 1 must have at least four days in the new year. A value of 1 means that the week containing 1 January is week 1, which is the US-style rule.

Many European regions, including Germany, France, the Netherlands and the United Kingdom, use the value 4 together with a Monday start. That is the ISO week. The United States and several other regions use a Sunday start with the value 1. So the two systems this calculator shows next to each other are the two most common ones.

The weekend moves too

The weekend is another regional variable. Most countries have a Saturday and Sunday weekend. In Saudi Arabia, Israel and Egypt the weekend is Friday and Saturday, so the working week runs from Sunday to Thursday. The United Arab Emirates moved its federal weekend to Saturday and Sunday on 1 January 2022, with a short working day on Friday.

This affects planning across borders. A supplier in one country and a buyer in another may share a week number but not a working week. A delivery "in week 47" has a different number of shared working days for them, so deadlines that depend on a handover should be given as dates.

Other calendars, same weeks

Several countries use a different calendar for religious or traditional purposes, such as the Islamic, Hebrew or Chinese calendar, or count years in another era, such as the Buddhist era in Thailand or the Japanese imperial eras. The seven-day week is shared between these calendars, but the numbering of weeks within a year belongs to the Gregorian civil calendar.

This means that ISO week numbers are usable everywhere, even in places where daily life follows another calendar. The ISO week-year is a Gregorian year, and the calculator on this site works with Gregorian dates for the years 1 to 9999.

Why a single date can have several numbers

Because of all these rules, a single date can carry a different week number depending on who counts. Sunday 25 October 2026 is in week 43 under the ISO rule, in week 44 under the US rule that starts the week on Sunday, and in week 43 under the Monday-start rule that counts the week containing 1 January as week 1.

Near New Year the differences are larger. Friday 1 January 2027 is in week 53 in the ISO system and in week 1 in the US system. Anyone who works with partners abroad should treat a bare week number as incomplete information.

Week numbers in software defaults

Programs differ in what they do if nobody chooses a rule. Some spreadsheet functions default to the US rule, some programming libraries default to the ISO rule, and some databases follow the setting of the server. A report that is produced in two systems can show different week numbers for the same data without any error in either.

The lesson for teams is to set the rule explicitly in every tool and to test a few dates near New Year. A week column that shows 53 for 1 January 2027 follows the ISO rule. A column that shows 1 follows the US rule.

A practical rule for international work

When you work across countries, say which rule you use. The easiest wording is "ISO week, Monday to Sunday". Then add the date range for important weeks, and the year with the week number. With those three details, partners in any country can read a week number the same way.

If your partner uses a different rule, the calculator can translate. Enter a date and read both the ISO number and the US number from the rows below the result, or enter the partner's week and year to see which dates they mean. A few seconds of checking is cheaper than a missed delivery.