Date & time
Week number calculator
Week numbering looks obvious until the turn of the year, where the ISO rule does something most people do not expect.
The ISO 8601 rule
The international standard defines week numbering with two rules:
- Weeks run Monday to Sunday.
- Week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January.
The Thursday rule exists to settle the boundary sensibly: it means week 1 is always the week with the majority of its days in the new year.
Why dates can belong to a different year
This is the part that surprises people. Because of the Thursday rule, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next.
1 January 2027 falls on a Friday. The first Thursday of 2027 is 7 January, so week 1 of 2027 begins Monday 4 January. That leaves 1-3 January 2027 in week 53 of ISO year 2026. A dated report filed under "2027 week 1" would be missing those days.
This is why ISO defines a separate week-year, shown above whenever it differs from the calendar year. Any system that stores week numbers should store the week-year alongside them, or January data will be misfiled every few years.
52 or 53 weeks
Most years have 52 ISO weeks. A year has 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday. That happens roughly every five to six years — 2020 and 2026 both have 53 weeks.
Payroll, retail reporting and manufacturing schedules built on a fixed 52-week assumption break in those years. The retail industry's common workaround is the 4-4-5 calendar, which accepts an occasional 53rd week explicitly rather than pretending it does not exist.
Other numbering systems
ISO 8601 is not universal. The North American convention typically starts weeks on Sunday and defines week 1 as the week containing 1 January, which produces different numbers for the same dates — often off by one, and a partial week 1 at the start of the year.
Islamic, Hebrew and other calendars number differently again. If you are exchanging week numbers with anyone, especially across borders, say which system you mean. "Week 35" alone is ambiguous.
Common questions
How is the ISO week number defined?
Weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of the year — equivalently the week containing 4 January. That guarantees week 1 has most of its days in the new year.
Why does 1 January sometimes fall in week 52 or 53?
Because week 1 is anchored to the first Thursday. If 1 January is a Friday, Saturday or Sunday, those days belong to the final week of the previous ISO year, and week 1 starts the following Monday.
How many weeks are in a year?
Usually 52. A year has 53 ISO weeks when it begins on a Thursday, or when it is a leap year beginning on a Wednesday — roughly every five to six years.
Is the US week number the same as ISO?
No. The common North American convention starts weeks on Sunday and puts 1 January in week 1, which often gives a number one higher than ISO and creates a partial first week. Always state which system you are using.