How many weeks are in a year?
Why a year is about 52 weeks and why some ISO years have week 53.
The arithmetic answer
A common year is 52 weeks plus one day. A leap year is 52 weeks plus two days. That remainder is why weekdays shift between successive years.
ISO week numbering
ISO weeks begin on Monday. Week 1 is the week containing January 4, which is also the first week with at least four days in the new year.
When week 53 appears
An ISO week-numbering year can have 53 weeks when its calendar alignment creates an additional full ISO week. This is a property of the week-year, not simply of every leap year.
Related year guides
- Leap years explained — Learn the Gregorian leap-year rule and why February sometimes has 29 days.
- Calendar quarters explained — See which months belong to Q1, Q2, Q3 and Q4.
- How many working days are in a year? — Estimate weekdays, then account for holidays and leave.