Weeks-in-Year Calculator

Worked example

2020 has 53 ISO weeks. Its December 28 falls in week 53, which is the defining test for a long ISO year, since a year has 53 weeks exactly when December 28 belongs to that final week. Most years have exactly 52 weeks of 7 days, 364 days total. 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, most often when the year starts on a Thursday, or on a Wednesday in a leap year.

Common mistakes

• Assuming a 53-week year happens on a fixed schedule, such as every 6 years exactly. The pattern depends on both the weekday January 1 falls on and whether the year is a leap year, so the actual gap between 53-week years varies.
• Confusing an ISO week count with a payroll or fiscal calendar's own week-numbering convention. A 4-4-5 fiscal calendar handles long years differently than ISO 8601 does, so the two systems will not always agree on which years are long.

Limitations

This reflects the ISO 8601 week system only. It does not represent a payroll or fiscal calendar's own week-numbering convention, which can define a long year differently.