Weeks in a Year
Explore week-based calculations. Find out how many weeks are in any year, look up ISO week numbers, and determine dates from day-of-year numbers.
Current Year at a Glance
Year
Total Weeks
Current Week
Weeks Remaining
Weeks in Any Year
Week Number Lookup
Day of Year to Date
Year Overview
Understanding ISO Week Numbering
The ISO 8601 standard defines a week numbering system used internationally. Under this system, weeks begin on Monday and week 1 is the week containing the year's first Thursday. This ensures that week 1 always contains January 4.
Most years have 52 ISO weeks. However, years where January 1 is a Thursday (or Wednesday in leap years) have 53 weeks. These "long years" occur roughly every 5 to 6 years.
Interesting facts: A year always has either 52 or 53 ISO weeks — never more, never less. The pattern of 52/53-week years repeats every 400 years, during which there are exactly 71 years with 53 weeks.
Frequently Asked Questions
A regular year has 365 days, which is 52 weeks and 1 day. So there are 52 full weeks plus one extra day. In ISO week numbering, most years have 52 weeks, but some years (called "long years") have 53 ISO weeks.
ISO 8601 defines a week as starting on Monday and ending on Sunday. Week 1 of a year is the week that contains the first Thursday of January (equivalently, the week containing January 4). This means some days at the beginning of January may belong to the last week of the previous year, and some days at the end of December may belong to week 1 of the next year.
A year has 53 ISO weeks if January 1 falls on a Thursday, or if December 31 falls on a Thursday. In a leap year, the year has 53 weeks if January 1 is on a Wednesday or Thursday. On average, a 53-week year occurs about every 5.6 years.