Week Number of the Year Calculator
This calculator tells you which week of the year and which day of the year any date falls on. Type in a date and it instantly returns the ISO 8601 week number, the day of the year counted from 1 January, the weekday and how many days are left until the end of the year. It is the tool you need when a spreadsheet, payroll system, shipping schedule or project plan refers to work by "week 32" instead of a calendar date, or when you simply want to know how far through the year a given day is. Everything updates live as you change the date, so you can scan an entire month in seconds. The calculator opens on today so you always start from the current week.
ISO week number
Day of the year
Days remaining this year
Day of the week
Follows ISO 8601: weeks start on Monday and week 1 is the one containing the first Thursday of the year.
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How it works
The week number uses the ISO 8601 standard, which is the definition most of Europe, business software and calendars rely on. Under ISO 8601 a week always starts on Monday and week 1 is the week that contains the year's first Thursday — equivalently, the week holding 4 January. Because of this rule the first days of January can belong to week 52 or 53 of the previous year, and late December can fall in week 1 of the next year. The day of the year is a straight count from 1 January (day 1) to 31 December, giving 365 in a normal year and 366 in a leap year, when 29 February pushes every later day forward by one. A year is a leap year when it is divisible by 4, except centuries not divisible by 400.
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Preguntas frecuentes
- What is an ISO week number?
- An ISO week number identifies a week of the year according to the ISO 8601 standard, from 1 to 52 or 53. Weeks always run Monday to Sunday, so an ISO week number is an unambiguous way to refer to a week that works the same across countries and software.
- Why is week 1 defined by the first Thursday?
- ISO 8601 defines week 1 as the week containing the year's first Thursday, which is the same as the week containing 4 January. Anchoring on Thursday guarantees that week 1 always has the majority of its days (at least four) in the new year, avoiding a "week 1" made mostly of the previous December.
- How many weeks are in a year?
- Most years have 52 ISO weeks. A year has 53 weeks when it starts on a Thursday, or on a Wednesday if it is a leap year. That happens roughly every five to six years, for example 2015, 2020 and 2026 all have 53 ISO weeks.
- What does "day of the year" mean?
- The day of the year is the position of a date counted from 1 January, where 1 January is day 1 and 31 December is day 365 (or 366 in a leap year). For example, 1 March is day 60 in a normal year and day 61 in a leap year because February has an extra day.
- What is a leap year and why does it matter here?
- A leap year has 366 days instead of 365 because of the extra day, 29 February. A year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. Leap years shift the day-of-year count for every date after February by one.