Week Number Calculator - ISO Week Number in Year
Calculate the current ISO 8601 week number of the year for any given date. Supports week numbering for project planning, payroll cycles, academic scheduling, and reporting periods. Includes week start day configuration (Monday or Sunday).
Week Numbering Standard
ISO Week = (Ordinal Day + 10 - Weekday) ÷ 7Variables:
- ISO WeekWeek number 1-52 (or 53)Week number 1-52 (or 53)(e.g.: Week 15)
- Ordinal DayDay number in the year (1-366)Day number in the year (1-366)(e.g.: Day 100)
- WeekdayMonday=1, Sunday=7Monday=1, Sunday=7(e.g.: Thursday = 4)
How to Use the KalkuLab ISO Week Number Calculator
- 1
Enter a Date
Input the date you want to check.
- 2
View Results
The ISO week number and day of the week are displayed.
Examples
Which Week in January?
Problem:
What ISO week number is January 15, 2024?
Solution:
- 1.January 15, 2024 = day 15 of the year
- 2.January 15, 2024 = Monday
- 3.Week number: (15 + 10 - 1) ÷ 7 = 24/7 ≈ 3
Result:Week 3 of 2024
January 15, 2024 is a Monday in ISO week 3.
End of Year
Problem:
What ISO week is December 31, 2024?
Solution:
- 1.2024 is a leap year (366 days)
- 2.Dec 31 = day 366
- 3.Dec 31, 2024 = Tuesday
Result:Week 1 of 2025
December 31, 2024 belongs to the first week of 2025 under ISO rules.
Frequently Asked Questions
What is the ISO 8601 standard?
ISO 8601 is the international standard for date and time representation. For weeks: Monday is the first day, and week 1 is the week containing January 4 (or the first Thursday of the year).
Why can January 1 fall in week 52 or 53 of the previous year?
Under ISO rules, a week belongs to the year in which most of its days fall. If January 1 falls on Friday–Sunday, that week still belongs to the previous year.
How many weeks are in a year?
Most years have 52 weeks. Some years have 53 weeks (a 'long year'), occurring roughly every 5–6 years.
Does Indonesia use the ISO standard?
Indonesia officially uses ISO standards for international business. In everyday calendars, Sunday is often treated as the first day (US convention).