Week Number Calculator ID
Find the ISO-8601 week number for any date, plus the day of the year and days remaining.
How it works
Pick a date and the calculator returns its ISO-8601 week number and week-year, the weekday, the day of the year and how many days are left in the year. It defaults to today.
Frequently asked questions
What is a week number?
It is the position of a week within the year, from 1 to 52 or 53. The ISO 8601 standard defines it precisely.
When does an ISO week start?
On Monday. ISO 8601 weeks always run Monday through Sunday.
How is week 1 defined?
It is the week containing the year’s first Thursday, which is the same as the week containing January 4.
Why do some years have 53 weeks?
Because a year is 52 weeks plus a day or two. A year has 53 ISO weeks when it begins on a Thursday, or a Wednesday in a leap year.
Can early January be in last year’s week?
Yes. If January 1–3 fall before the first Thursday, they belong to the final week of the previous year under ISO rules.
Where are week numbers used?
In business scheduling, manufacturing, supply chains and payroll, where referring to a week is clearer than a specific date.
What a week number is
A week number tells you which week of the year a date falls in, from 1 up to 52 or 53. The international standard, ISO 8601, is widely used in business, manufacturing and project planning to refer to weeks unambiguously across countries.
How ISO weeks work
Under ISO 8601, weeks start on Monday, and week 1 is the week containing the year’s first Thursday — equivalently, the week that contains January 4. This means the first few days of January can sometimes belong to the last week of the previous year.
Why 52 or 53 weeks
Because 365 days is 52 weeks plus one day (or two in a leap year), most years have 52 ISO weeks but some have 53. A year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year.
Week numbers are common in supply chains, payroll and scheduling — saying "ship in week 38" is clearer across teams and time zones than a specific date, and everyone using ISO weeks lands on the same range of days.
Results are estimates for general guidance in Indonesia and may not reflect the latest local rates, fees or rules. Check official sources before making decisions.