📅 Week Number Calculator
Find the ISO week number for any date and see week start and end dates instantly.
What is this tool?
The Week Number Calculator is a free online tool that determines the ISO 8601 week number for any given date. ISO week numbering is the international standard used across Europe and much of the world for scheduling, project management, and business planning. Under this system, each year has 52 or 53 weeks, and weeks always begin on Monday. This tool is especially useful for professionals who work with weekly schedules, production planning, academic calendars, or reporting periods that follow ISO standards. Enter any date, and the calculator instantly shows you the week number, the start date (Monday) and end date (Sunday) of that week, and whether the week belongs to the previous or next year for dates near January 1st. It also displays the day of the year (1 to 365 or 366), making it a versatile companion for date-based planning.How it works
This tool uses the ISO 8601 week date standard. The algorithm works by first identifying the Thursday of the week containing the input date, which determines the ISO year. Week 1 of any year is defined as the week containing the first Thursday of January, or equivalently, the week containing January 4th. This ensures consistency even when January 1st falls on a weekend. The calculation involves finding the day of the week for the input date, then adjusting to find the nearest Thursday. From there, the tool counts how many Thursdays have passed since the start of the ISO year to determine the week number. Week start (Monday) and end (Sunday) dates are derived by simple offset calculations from the input date based on its weekday. All computations are performed with JavaScript's Date object in your browser.How to use
- Enter any date in the date picker field.
- Click the Calculate button to process the date.
- View the ISO week number displayed in the result area.
- Check the week start (Monday) and end (Sunday) dates for that week.
- See additional info like day of the year and total days remaining in the year.
Frequently Asked Questions
Frequently Asked Questions
What is ISO 8601 week numbering?
It is an international standard where weeks start on Monday, and Week 1 is the week containing the first Thursday of the year.
Can a date near New Year belong to a different year's week?
Yes. For example, December 31 might be in Week 1 of the next year, or January 1 might be in Week 52 or 53 of the previous year.
How many weeks are in a year?
Most years have 52 weeks, but some years have 53 weeks. This occurs in years where January 1st is a Thursday (or Wednesday in leap years).
Does this work for historical dates?
Yes, you can calculate week numbers for any date in the past or future using the ISO standard.
Tips & Advice
ISO week numbers are widely used in manufacturing, logistics, and European business calendars. When referencing week numbers in international communications, always clarify you mean ISO weeks, as some countries (like the US) use different week numbering systems where weeks start on Sunday and Week 1 contains January 1st. For project planning, knowing the exact Monday-to-Sunday date range of a week number prevents scheduling confusion.