ISO week numbers explained
By Vijay Jadhav
Week numbers are handy for planning at work, but two calendars can show different numbers for the same day. This guide explains the ISO system used on this site and how it compares with others.
The ISO 8601 rules
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year's first Thursday. Put another way, it is the week with January 4 in it.
- Every week belongs entirely to one year, so the first few days of January can belong to the last week of the previous year.
Why some years have 53 weeks
A year has 52 weeks and one or two extra days. When a year starts on a Thursday, or is a leap year starting on a Wednesday, those extra days add up to a 53rd week. 2026 is one of those years: it starts on a Thursday and has 53 ISO weeks.
US-style week numbers
Many US calendars start weeks on Sunday and simply call the week containing January 1 week 1. That system can be one number ahead of ISO for much of the year. If a colleague's week number does not match yours, this is usually the reason.
In spreadsheets
In Excel and Google Sheets, =ISOWEEKNUM(date) gives the ISO week, and =WEEKNUM(date) gives the US-style week.