ISO Week Number Calculator

Side Note — Why Thursday Decides the Week Number

The ISO 8601 rule for week numbering can feel counterintuitive at first: which year (and which week number) a week belongs to is decided not by its start (Monday) but by its Thursday. The logic is that a week belongs to whichever year contains the majority of its seven days. Since Thursday is the fourth day of a Monday-starting week — exactly the midpoint — checking which year Thursday falls in is a simple shortcut for determining which year holds the majority of that week's days.

A side effect of this rule is that the week containing January 1st isn't always "week 1." If January 1st falls on a Friday, Saturday, or Sunday, that week's Thursday actually falls in the previous year, so January 1st gets counted as part of the previous year's final week (week 52 or 53). Conversely, if December 31st falls on a Monday, Tuesday, or Wednesday, the week containing it becomes week 1 of the following year.

Whether a year has 52 or 53 weeks is also a consequence of this Thursday rule. A common year (365 days) is 52 weeks plus one extra day, and a leap year (366 days) is 52 weeks plus two extra days — and whichever year that leftover day's Thursday falls into determines whether the year needs a 53rd week. This happens roughly 71 times every 400 years, or about once every 5 to 6 years.

Because ISO week numbers (Monday-based) differ from the US-style calendar week numbering (Sunday-based) used in some spreadsheets and calendars, the same date can be reported as a different week number depending on the convention. When week numbers matter for cross-border business — such as manufacturing lot codes or international logistics — it's worth explicitly stating that ISO 8601 is the convention being used.