Week Number Calculator

Find the week number for any date with our free online calculator. Perfect for project planning, scheduling, and date calculations.

Features

• Calculate week number for any date
• Multiple week numbering systems:
  • ISO week numbers (ISO-8601)
  • Simple week numbers
• Current week number display
• Date picker interface
• Week start options:
  • Monday (ISO standard)
  • Sunday (US standard)
• Mobile-friendly interface
• No registration required

How to Use

1. Select a date using the calendar
2. Choose your preferred week numbering system
3. View the week number instantly
4. See additional date information
5. Switch between week start options

Understanding Week Numbers

Week numbers can be confusing because there are different systems for counting them. The most common are ISO week numbers and simple week numbers, and they work differently. Understanding which system you're using matters when you're planning or communicating about dates.

ISO week numbers follow the ISO-8601 standard, which is used internationally. Weeks start on Monday, not Sunday. Week 1 is defined as the first week that contains at least 4 days of January. This means week 1 might actually start in December if January 1st falls late in the week. Also, weeks can belong to different years—the last few days of December might be in week 1 of the next year. Years always have either 52 or 53 weeks in the ISO system.

The ISO system is used in Europe and many international businesses. It's standardized, which makes it useful for coordinating across borders. When companies schedule projects or report on weekly metrics, they often use ISO week numbers. It's consistent and predictable once you understand how it works.

Simple week numbers are more straightforward. Week 1 starts on January 1st, regardless of what day of the week that is. If January 1st is a Wednesday, then week 1 runs from Wednesday to Tuesday. All weeks belong to the current calendar year. There are usually 52 full weeks plus a partial week at the end of the year. This system is simpler to understand but less standardized.

In the US, week numbering often starts on Sunday, which is different from the ISO Monday start. When you're working with different systems, it's important to know which one you're using. A date might be in week 10 in one system and week 11 in another, which can cause confusion if you're not careful.

Project management uses week numbers constantly. When planning sprints, milestones, or delivery schedules, referring to week numbers makes communication easier. "We'll deliver this in week 15" is clearer than "around the middle of April." Week numbers provide a consistent way to refer to time periods throughout the year.

Business planning relies on week numbers for reporting and scheduling. Fiscal weeks, production schedules, and inventory cycles often use week numbers. Weekly reports are numbered, making it easy to track trends and compare periods. Understanding week numbers helps with business planning and reporting.

International business needs to account for different week numbering systems. A European company might use ISO week numbers, while a US company uses a Sunday-start system. When coordinating across borders, being aware of these differences prevents scheduling conflicts and misunderstandings.

Academic planning uses week numbers for semesters and course schedules. Many schools organize their academic calendars by week numbers. Assignments might be due in "week 7" or exams scheduled for "week 14." Understanding week numbers helps students and teachers stay organized throughout the term.

Event planning uses week numbers for scheduling and coordination. When planning conferences, workshops, or other events that span multiple dates, using week numbers can simplify communication. "The event runs from week 20 to week 22" is easier to communicate than listing specific dates.

This calculator makes it easy to find week numbers for any date. Select a date, choose your preferred numbering system, and get the week number instantly. No need to count weeks manually or figure out which system is being used. Whether you're planning projects, coordinating with international teams, or just organizing your schedule, this tool gives you accurate week numbers.

Common Applications

Project Management

• Sprint planning
• Milestone tracking
• Delivery schedules
• Timeline organization

Business Planning

• Fiscal weeks
• Production schedules
• Reporting periods
• Inventory cycles

Personal Organization

• Academic planning
• Event scheduling
• Holiday planning
• Task management

Why Use Our Calculator

• Instant calculations
• Multiple systems
• Clear interface
• Works offline
• Free to use
• No ads or popups