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Week number calculator

Europe and most of the world number weeks the ISO 8601 way: weeks start on Monday and week 1 is the one that contains the first Thursday of January. The United States and much of Latin America count from the week of 1 January instead, starting on Sunday. This calculator gives both and works in either direction.

ISO week43
Monday to Sunday
October 19, 2026 → October 25, 2026
Week-numbering year
2026
US week number (starting Sunday)
43
Weeks in that year
53
Day of the year
296
Days left in the year
69
Quarter
4

The formula

ISO week = floor((day of year − weekday + 10) ÷ 7), with Monday = 1 and Sunday = 7. A result of 0 belongs to the last week of the previous year; 53 becomes week 1 of the next year when that year starts on a Monday to Thursday. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year.

Worked example

23 October 2026 falls in ISO week 43, which runs from Monday 19 to Sunday 25 October. It is day 296 of the year, with 69 days left. 2026 starts on a Thursday, so it has 53 weeks.

How to use it

  1. 1Choose whether you start from a date or from a week number.
  2. 2For a week, enter the year and the number from 1 to 53.
  3. 3Use the ISO number for European calendars, payroll and logistics; use the US number for American calendars and many Latin American agendas.

Questions people ask

Why is 1 January sometimes in week 52 or 53?

Because ISO week 1 is the week with the first Thursday. When 1 January falls on a Friday, Saturday or Sunday, those days still belong to the last week of the previous year. 1 January 2027, a Friday, is in week 53 of 2026.

Which week number does Excel use?

ISOWEEKNUM returns the ISO number. WEEKNUM with no second argument returns the US-style number, starting on Sunday with the week of 1 January as week 1.

Which countries use ISO weeks?

Almost all of Europe, plus many companies worldwide for planning. Spain, France and Belgium use them; the United States, Canada, Mexico and much of Latin America usually show Sunday-start weeks.

Last reviewed: 2026-10-23