Week Numbers Explained
Under ISO 8601, weeks run Monday to Sunday and week 1 is the week containing the first Thursday of January. That is why early January sometimes belongs to week 52 or 53 of the previous year, and why some years have 53 weeks.
The rule in one sentence
Week 1 of a year is the week that contains the first Thursday of January — equivalently, the week containing 4 January, and the first week with the majority of its days in the new year.
Because weeks start on Monday, up to three days at the start of January can belong to the final week of the previous year, and up to three days at the end of December can belong to week 1 of the next.
Why some years have 53 weeks
A common year is 52 weeks and one day; a leap year is 52 weeks and two days. Those spare days accumulate, and a year gets a 53rd ISO week when it starts on a Thursday, or when it is a leap year starting on a Wednesday.
This matters more than it sounds. Payroll, retail reporting and manufacturing schedules built on 52-week years need an explicit rule for the extra week, and a printed calendar with the week column makes the anomaly visible before it causes a problem.
Advertisement
Where week numbers are used
Week numbering is standard across northern Europe in everyday conversation — a Swedish or German colleague will say week 32 rather than early August. It is also standard in manufacturing, logistics, construction scheduling and much of project management software.
Software teams running fixed sprints and retailers planning promotions both tend to work in week numbers regardless of country.
Adding week numbers to a printed calendar
Turn on the week number option and a narrow column appears on the left of the grid showing the ISO week each row belongs to. Set the week start to Monday at the same time, so the printed rows match the ISO week boundaries exactly.
With a Sunday start, the number shown is still the correct ISO week for the dates in that row, but a single row will straddle two ISO weeks.
Other week numbering systems
Not everyone uses ISO. Some North American systems number the week containing 1 January as week 1 and start weeks on Sunday, which can give a different number for the same date. Broadcast calendars used in television advertising use their own Monday-start scheme.
If you exchange week numbers with anyone outside your organisation, state which system you mean once. ISO 8601 is the safest default because it is the only one defined by an international standard.
Frequently asked questions
- How many weeks are in a year?
- 52 in most years and 53 in years that begin on a Thursday, or leap years that begin on a Wednesday.
- Why does January start in week 52?
- Because ISO week 1 is the week containing the first Thursday of January. Days before that week belong to the last week of the previous year.
- Do week numbers start on Sunday or Monday?
- ISO 8601 weeks start on Monday. Set the week start to Monday for the printed rows to match.
Advertisement