Week number calculator

How the ISO week number is calculated

ISO 8601, the international date standard used across Europe and in most business software, defines weeks as running Monday to Sunday. Week 1 of a year is the week containing that year's first Thursday, which is the same as the week containing 4 January. To find the number for any date, the calculator moves to the Thursday of the same week, counts how many days into the year that Thursday is, divides by seven, rounds down and adds one.

Worked example: 22 October 2026 is a Thursday, and it is day 295 of the year. (295 − 1) ÷ 7 = 42, plus one gives week 43, running from Monday 19 October to Sunday 25 October 2026. Because the Thursday rule decides the year, dates near New Year can belong to the neighbouring year: Friday 1 January 2027 still falls in week 53 of 2026.

A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday; every other year has 52.

ISO weeks versus other week numbers

Not every calendar agrees. Many US calendars and spreadsheet defaults start weeks on Sunday and call the week containing 1 January week 1, so the same day can carry a different number; the result above shows both. When you share a week number with suppliers or colleagues in other countries, say which system you mean, or write it in ISO form such as 2026-W43, which also removes doubt about the year.

In spreadsheets, Excel's ISOWEEKNUM function and WEEKNUM with return type 21 follow ISO 8601, while plain WEEKNUM uses the Sunday-start rule. Payroll periods, retail calendars such as 4-4-5 accounting years and school timetables may use their own numbering, so check before relying on week numbers for deadlines or invoices.