The ISO Week Date Format: 2026-W43-4

What the format looks like

ISO 8601 defines a week date as a year, a week number and a weekday. The extended form is 2026-W43-4. It has four parts: the week-year 2026, a capital letter W, the two-digit week number 43, and a single digit for the weekday, where 1 is Monday and 7 is Sunday. The example is Thursday of week 43 of 2026, which is 22 October 2026.

A week without a weekday is written 2026-W43. It names the whole week from Monday 19 October to Sunday 25 October 2026. The basic form without hyphens is 2026W434 for a day and 2026W43 for a week. It is the same information with fewer characters, and is used where space is tight, such as in file names.

The week-year is not always the calendar year

The first part of a week date is the ISO week-year, not the year of the calendar date. This is the source of the most frequent mistake. Friday 1 January 2027 is written 2026-W53-5, because that day belongs to week 53 of 2026. Monday 29 December 2025 is written 2026-W01-1, because it belongs to week 1 of 2026.

If you build a week date from the year of the date and the ISO week number, you get 2027-W53-5 for 1 January 2027, a week that does not exist. The week-year has to be taken from the Thursday of the week, or from a function that returns it together with the week number.

Two digits and capital letters

The week number always has two digits, with a leading zero: 2026-W05, not 2026-W5. The letter W is capital. This is not just taste: fixed width and a fixed letter make the string sortable and easy to parse, and strings in this form sort in chronological order, also across New Year.

The weekday digit runs from 1 to 7 with Monday as 1. That differs from several programming languages that number the days from 0 or start with Sunday. Always convert explicitly. A value of 7 means Sunday, never 0.

Converting between week dates and calendar dates

To convert a week date to a calendar date, find the Monday of week 1 of the week-year, which is the Monday on or before 4 January, add 7 times the week number minus 1 days, and add the weekday digit minus 1. For 2026-W43-4: week 1 starts on 29 December 2025, plus 42 weeks is 19 October 2026, plus 3 days is 22 October 2026.

To convert the other way, find the Thursday of the week of the date. Its year is the week-year, and the week number is the number of weeks from 1 January of that year. The weekday digit is the weekday of the date. The calculator on this page does this for you, and the second tab accepts a year and week number.

Programming languages have functions for it. Python has date.fromisocalendar(2026, 43, 4) and the format codes %G, %V and %u for week-year, week and weekday. PostgreSQL has the IYYY, IW and ID format codes. Java's IsoFields class provides the week-based year. JavaScript has no built-in support, so libraries or hand-written code are needed.

Where week dates are used

The week date is used where the week is the main unit: production schedules, project plans, delivery commitments and statistical reports. Government statistics, especially in Europe, often publish weekly series labelled by week-year and week, such as 2026-W43. It also appears in some file name conventions for weekly exports.

It is less common in general-purpose date fields. The profile of ISO 8601 that is used on the web and in many interchange formats, RFC 3339, does not include week dates. A form that accepts only calendar dates will reject 2026-W43-4. If you want to exchange week dates, state the format.

Ordinal dates and calendar dates

ISO 8601 has two more date forms next to the week date. The calendar date is 2026-10-22. The ordinal date counts days within the year: 2026-295, because 22 October is the 295th day of 2026. The calculator shows the ordinal number as the day of the year.

The three forms describe the same day. 22 October 2026 is 2026-10-22 as a calendar date, 2026-295 as an ordinal date and 2026-W43-4 as a week date. Which one you choose depends on the use: the calendar date for people, the ordinal date for systems that count days, and the week date for week-based plans.

Rules of thumb

Always include the week-year and keep it in four digits. Always write the week number with two digits. Use 1 to 7 for the weekday. Validate week 53: it is allowed only in long years, so 2027-W53 is invalid and 2026-W53 is valid.

When you read a week date that has been written by hand, check the first days of January and the last days of December. If you see 2027-W53 or 2026-W01 for a date in the middle of the year, the string was built wrongly. A week date that passes these checks is as precise as a calendar date and shorter than a date range.