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ISO 8601 · Reference

ISO 8601 Week Numbering Standard Explained

ISO 8601 defines week numbering used internationally in software, finance, and European business. Monday-start weeks; week 1 contains the first Thursday.

ISO 8601 week numbering is the international standard that defines weeks as Monday-to-Sunday blocks numbered 1 to 52 or 53 per year, where week 1 is the week containing the first Thursday of January. Software, EU business, and finance rely on this rule for consistent week dates.

What is ISO 8601?

ISO 8601 is the international date and time standard published by the International Organization for Standardization (ISO). The current edition is ISO 8601-1:2019 and ISO 8601-2:2019, titled "Date and time — Representations for information interchange" (see https://www.iso.org/iso-8601-date-and-time-format.html). The standard covers calendar dates, ordinal dates, week dates, times of day, time intervals, durations, and recurring intervals. It exists to remove ambiguity from date exchange across languages, locales, and systems.

The week-date portion of ISO 8601 specifies three things: weeks start on Monday, every week belongs to exactly one week-based year, and week 1 is defined by a single deterministic rule (covered next).

How ISO 8601 week numbers are calculated

Week 1 of any ISO year is the week containing the first Thursday of January. Equivalently, week 1 is the week containing 4 January. Because weeks always run Monday through Sunday under ISO 8601, this rule fixes every other week unambiguously.

For example, 4 January 2027 is a Monday, so the week of Monday 4 January 2027 is 2027-W01. The notation is YYYY-Www-D, where D is the ISO weekday (Monday is 1, Sunday is 7).

A date near year boundaries can sit in a week-based year different from its calendar year. That concept is called the ISO week-based year. The full algorithm and code samples live on a separate page: see How ISO 8601 Week Numbers Are Calculated.

Examples

Three concrete cases show the rule at work.

  • 28 May 2026 is a Thursday in week 22, so its ISO week date is 2026-W22-4.
  • 1 January 2023 is a Sunday and falls in ISO week 52 of week-based year 2022, written 2022-W52-7.
  • 31 December 2026 is a Thursday in ISO week 53 of 2026, written 2026-W53-4, because 2026 is a 53-week year.

Common edge cases

Three patterns trip up developers and analysts.

  • 53-week years. An ISO year has 53 weeks when 1 January is a Thursday, or when it is a Wednesday in a leap year. Recent and upcoming 53-week years are 2015, 2020, 2026, 2032, 2037, 2043, and 2048. The pattern repeats roughly every 5–6 years.
  • December dates in the next ISO year. If 29, 30, or 31 December falls on Monday, Tuesday, or Wednesday, those dates belong to week 1 of the following ISO year.
  • January dates in the prior ISO year. If 1, 2, or 3 January falls on Friday, Saturday, or Sunday, those dates belong to the last week (52 or 53) of the previous ISO year.

Software and tooling support

Major languages and databases implement ISO 8601 week numbering directly.

  • PostgreSQL: EXTRACT(week FROM date) returns the ISO week; EXTRACT(isoyear FROM date) returns the ISO week-based year.
  • Python: date.isocalendar() returns (iso_year, iso_week, iso_weekday).
  • JavaScript: the Temporal API exposes Temporal.PlainDate.from("2026-05-30").weekOfYear; the legacy Date object requires a manual algorithm.
  • Microsoft Excel: =ISOWEEKNUM(date) (Excel 2013 and later); =WEEKNUM(date, 21) returns the same value.
  • SQL Server: DATEPART(iso_week, date) returns the ISO week.
  • Java: WeekFields.ISO.weekOfWeekBasedYear() from java.time.temporal.

When to use ISO 8601

Pick ISO 8601 when you need cross-border consistency. It is the right default for international data exchange, EU and Australian business reporting, financial systems, logistics, and almost every developer-facing API. Use it when teams across regions must agree on a single week number for a given date.

Avoid ISO 8601 when your audience expects Sunday-start weeks. US retail, US payroll, and consumer-facing Excel reports usually want the non-ISO WEEKNUM(date) default. Mixing standards inside one report causes silent off-by-one errors.

Related terms

Compare with other standards

US Week Numbering starts each week on Sunday and defines week 1 as the week containing 1 January. It is the default in Microsoft Excel's plain WEEKNUM function and dominates US payroll and retail calendars.

Australian Financial Year Week Numbering numbers weeks against the Australian financial year, which runs from 1 July to 30 June. The Australian Taxation Office, government reporting, and many local businesses use this calendar instead of the calendar-year-based ISO scheme.

FAQ

What is the first week of the year in ISO 8601? Week 1 is the week containing the first Thursday of January. Equivalently, it is the week containing 4 January. The week always runs Monday through Sunday under this rule.

When does week 53 happen? Week 53 happens when 1 January is a Thursday, or when 1 January is a Wednesday in a leap year. Recent 53-week years are 2015, 2020, and 2026; next are 2032 and 2037.

Does ISO 8601 use Monday or Sunday as the first day? ISO 8601 uses Monday as the first day of the week. Monday is weekday 1 and Sunday is weekday 7. This differs from the US convention, which treats Sunday as the first day.

Does Microsoft Excel support ISO week numbers? Yes. Excel 2013 and later include =ISOWEEKNUM(date), which returns the ISO 8601 week number. The formula =WEEKNUM(date, 21) returns the same value using the standard mode argument.

For the step-by-step algorithm and code, see How ISO 8601 Week Numbers Are Calculated.

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By Week Number Australia editorial team·Updated