WhenMathWeeks-in-Year Calculator
Most years have 52 ISO weeks; some "long" years have 53. Find out which for any ISO year.
Worked example
2020 has 53 ISO weeks: its December 28 falls in week 53, which is the defining test for a long ISO year. Most years have exactly 52 weeks of 7 days (364 days); the extra day or two each year (365 or 366 total) accumulates until it forms a 53rd week roughly once every 5 to 6 years.
Limitations
This reflects the ISO 8601 week system only, not a payroll or fiscal calendar's own week-numbering convention.
Frequently asked questions
- How often does a 53-week year happen?
- Roughly once every 5 to 6 years, though the exact pattern depends on leap years and which weekday January 1 falls on.