Week Number Calculator

Convert Dates and Week Numbers

If you’ve ever stared at a calendar trying to figure out which week a specific date falls into—or needed to plan something based on a week number—this tool is for you. Whether you're working with international teams, aligning schedules, or managing projects, this calculator helps you easily switch between calendar dates and week numbers.

What This Calculator Can Do

The week number converter handles two main tasks:

• Turn any calendar date into a week number
• Find the start and end dates for any given week number and year

It also supports three different systems for week numbering:

• ISO 8601: Used internationally, with weeks starting on Monday
• US/Canada: Weeks start on Sunday, and Week 1 always includes January 1
• Simple Monday system: A less strict system where Week 1 includes January 1 and weeks begin on Monday

Why You Might Want to Use It

Week numbers are standard in industries like logistics, software development, and retail. They’re also common in European work calendars and government reports. This calculator is especially helpful when:

• You're interpreting a project timeline like “Deadline: W23”
• You want to avoid timezone mistakes by using week ranges instead of exact dates
• You’re converting between formats used by coworkers, clients, or reporting Tools

Using the Calculator Step-by-Step

Option 1: Date to Week Number

1. Select “Date to Week Number” at the top of the settings.
2. Enter a date in the date field. The current date is filled in automatically when the page loads.
3. Pick your preferred week numbering system from the dropdown (ISO 8601, US, or Simple).
4. Click “Calculate Week Number”.

The result section will show:

• The week number
• The day of the week for the date you entered
• Total number of weeks in that year (based on the system you chose)
• A full list of dates for that week

Option 2: Week Number to Date

1. Select “Week Number to Date Range”.
2. Enter a year and a week number (1–53, depending on the system).
3. Pick the week system you’re using from the dropdown.
4. Click “Calculate Week Number”.

You’ll see the full date range for that week, along with a formatted breakdown just like in the other mode.

Extra Features You Might Miss at First Glance

Live Date and Week Info

In the bottom corner of the tool, you’ll see the current date and the current week numbers—both for ISO and US formats. These update every minute in the background.

Date Format Toggle

A button lets you switch between UK-style and US-style date formats. This only affects how the dates are shown (e.g., “11 June 2025” vs “June 11, 2025”), not the week number calculations.

Built-In Safety Checks

If you enter a week number that doesn’t exist in a given year (for example, Week 53 in a year that only has 52 weeks), the tool will let you know right away. It also checks that the year falls between 1900 and 2100.

Answers to Questions You Probably Have

Can a date belong to a different year’s week?

Yes—especially in the ISO system. For instance, December 30, 2024 falls into Week 1 of 2025 under ISO rules. The calculator handles this automatically and shows you the correct “week year.”

Why do some years have 53 weeks?

In ISO, a year can have 53 weeks if January 1st is a Thursday, or if it’s a leap year starting on a Wednesday. The tool figures this out based on the system and year you enter.

Does it account for different week starts?

Absolutely. Each week system has its own rules. ISO starts weeks on Monday, US on Sunday, and the Simple system also uses Monday. The tool adapts based on your selection.

Keep Your Calendar Clean and Your Weeks Straight

Whether you’re juggling deadlines, syncing with a global team, or just want a clearer sense of time, this calculator keeps week numbers and dates aligned without the guesswork. It’s a practical way to stay organized, no matter what system you or your team uses.