Week numbers are common in offices, factories, schools and logistics, especially in Europe: “the delivery is due in week 14” is shorter than spelling out a date range. Almost everyone who uses week numbers uses the ISO 8601 system, so it helps to know exactly how it works.
The rules
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year’s first Thursday. Equivalently, it is the week that contains 4 January.
- Every week belongs to exactly one year, so a year has either 52 or 53 weeks.
Because of the Thursday rule, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For example, Friday, January 1, 2027 falls in ISO week 53 of 2026.
Which years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. That happens 71 times every 400 years — roughly every five to six years. The next long years are 2026, 2032, 2037, 2043, 2048.
A quick way to find the week number
Take the Thursday of the week you are interested in. Count which week of its year that Thursday falls in — the ISO year is the year of that Thursday. Spreadsheets do this for you: =ISOWEEKNUM(A1) in Excel or Google Sheets.
ISO weeks vs. US-style week numbers
Some US software numbers weeks starting on Sunday and calls the week containing 1 January “week 1”. Those numbers can differ from ISO by one, especially around New Year. If you share schedules internationally, say “ISO week” to avoid confusion.
Week numbers on printable calendars
Every calendar on this site can show ISO week numbers in a narrow column beside each row — turn on “ISO week numbers” in the options. Week numbers pair well with a Monday week start; read more about choosing a week start. Try it on the 2026 UK calendar or the 2026 US calendar.