Week Number Calculator

What Is an ISO Week Number?

An ISO week number is a standardised way of identifying a specific week within a year, defined by the international date standard ISO 8601. Rather than dividing a year into months, the ISO week system divides it into 52 or 53 numbered weeks, each running from Monday to Sunday.

The week number calculator on DateAndDays.com.au tells you which ISO week any date falls in — an essential reference for businesses, project managers, and industries that operate on a weekly cycle.

How ISO Week Numbering Works

ISO 8601 defines week 1 as the week that contains the first Thursday of the year. This rule has several important consequences:

• The ISO week year begins on Monday and ends on Sunday.
• The first week of the year always contains 4 January.
• The last few days of December may belong to week 1 of the following year.
• The first few days of January may belong to week 52 or 53 of the previous year.

For example, 1 January 2023 fell on a Sunday. Under ISO rules, that date belonged to week 52 of 2022, not week 1 of 2023. ISO week 1 of 2023 began on Monday 2 January.

How Many Weeks Are in a Year?

Most years contain 52 ISO weeks. A year has 53 ISO weeks when it begins on Thursday or, in a leap year, on Wednesday or Thursday. The year 2026 has 53 weeks; the year 2025 has 52. You can find the week count for any year using a week number calculator.

Why ISO Week Numbers Matter

ISO week numbers provide a consistent way to refer to time periods that transcends month-boundary ambiguity. They are widely used in:

• Manufacturing and supply chain scheduling, where production runs and deliveries are planned by week number.
• Financial reporting, where weekly figures need to be comparable across years.
• Broadcasting and media, which often schedule programming by ISO week.
• Construction and project management, where Gantt charts use week numbers.
• Retail analytics, where same-week-last-year comparisons require precise week identification.

ISO Week Notation

ISO 8601 expresses week dates in the format YYYY-Www-D, where YYYY is the ISO year, ww is the two-digit week number, and D is the day of the week (1 = Monday, 7 = Sunday). For example, 2026-W08-5 represents Friday of ISO week 8 in 2026, which is 20 February 2026.

Week Numbers in Australia

Australian businesses frequently use ISO week numbers for reporting and scheduling. The financial year (1 July to 30 June) does not align neatly with ISO weeks, so Australian companies operating on ISO weeks may have partial weeks at the start of July and end of June. A week number calculator helps reconcile these boundaries.

Worked Examples

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