How to Write Week Numbers: W43, CW 43, KW 43

Many spellings, one idea

A week number can be written in many ways, and the choice depends on the language, the field and the tool. "Week 43" is the plain form. "Wk 43" and "W43" are short forms common in plans and charts. "CW 43" stands for calendar week and is common in international companies. "KW 43" is the German form, from Kalenderwoche, and is understood in many European industries. The ISO form is "2026-W43".

All of them mean the same thing if the writer and the reader use the same rule. The spelling does not tell you the rule. For that you need either a convention or a short note. The more formal the document, the more important the note.

The ISO form

The ISO 8601 form writes the year, a hyphen, the capital letter W and the two-digit week: 2026-W43. A weekday can be added as a digit from 1 to 7, as in 2026-W43-4 for Thursday. This form is unambiguous, because it contains the week-year and because the standard defines the rule. It also sorts correctly as text.

Use it in file names, databases, data exports, headers of weekly reports and anywhere that machines or people from several countries read the label. For file names the compact form 2026W43 is also valid.

Spellings in other languages

In German-speaking countries, "KW 43" or "43. KW" is standard in business. French plans write "semaine 43" or "S43", Spanish documents "semana 43" or "S43", Italian "settimana 43" or "sett. 43", Dutch "week 43" or "wk 43", Portuguese "semana 43" or "sem. 43", and Japanese "第43週". Swedish calendars use "v. 43" and "vecka 43".

The abbreviations are not always understood across borders. A US colleague may not know KW, and a German colleague may read "S43" as something else. In mixed teams, "week 43" or "W43" is the safest choice, and the ISO form removes the doubt.

Always add the year

A week number without a year is incomplete. Week 43 exists every year, and a plan that says "week 43" in a document that lives for twelve months can be read as last year's or next year's week. Write "week 43, 2026" or "2026-W43" in anything that will be read later.

Be careful with the year near New Year. A report for "week 1, 2027" starts on Monday 4 January 2027. A report for "week 1" that is written in December 2026 could mean the week that starts on 29 December 2025 if the author used last year's calendar. The week-year in the ISO form prevents the mix-up, because 2026-W53 and 2027-W01 are different weeks.

Ranges and dates

A range of weeks can be written as "weeks 43 to 46", "W43–W46" or "2026-W43 to 2026-W46". The end of the range is the Sunday of week 46, which is 15 November 2026, and the start is Monday 19 October 2026. A range that crosses New Year should show the years, as in "2026-W51 to 2027-W02".

For important deadlines, add the date in brackets. "Delivery in week 47 (16 to 22 November 2026)" tells the reader what you mean even if they use another rule. The calculator gives the date range with one click, so it costs nothing to include it.

Formatting rules for documents

Choose one form and keep it in the whole document. Do not mix "W43", "KW43" and "week 43" in one table. Use a non-breaking space between the abbreviation and the number if the text might wrap, so that "KW" is not left alone at the end of a line.

In spreadsheets, store the week as two columns, the week-year and the week number, or as the ISO text, rather than as a single number. A single number cannot carry the year, and sorting a column of plain numbers puts week 1 of every year together. In databases, an integer such as 202643 is compact and sortable, but make sure the year part is the week-year, not the year of the date.

Writing week numbers in code and data

In code, avoid storing a week as free text. Store the week-year and the week number as integers, or the ISO string, and convert to display text at the last step. A display such as "KW 43" is a presentation choice, and it changes with the language of the user. The data should not change with it.

If you export data to a tool that expects a different form, convert at the border and name the column clearly, for example iso_week or iso_week_year. A column called week alone is a warning sign, because nobody knows which rule produced it.

A short guide

For informal use: "week 43". For plans and charts: "W43" with the year in the heading. For international documents: "CW 43" or "week 43 (ISO)". For data and files: "2026-W43". For deadlines: week number plus date range.

Whatever you choose, test it on a date near New Year. If your label for 1 January 2027 is "2026-W53" or "week 53 of 2026", it is correct. If it says "2027-W53", the label is built from the wrong year, and the week does not exist.