What is an ISO week?
ISO 8601 defines week 1 as the first week of the year containing at least 4 days (or containing the first Thursday).
How to Use Week Number Calculator
Step 1: Select Any Date
Pick a date on the calendar or view today.
Step 2: Get Week Info
Instantly see the week number (1–53), start Monday, and end Sunday.
Key Features & Capabilities
- Standard ISO 8601 week calculation
- Total weeks in the selected year (52 or 53)
- Percentage of year elapsed through the current week
Frequently Asked Questions
Why do some years have 53 weeks?
Years that start on a Thursday (or leap years starting on Wednesday) have 53 ISO weeks because 52 weeks only equal 364 days.
Related Tools
- Day of the Year Calculator — Find the ordinal day number (1–366) and remaining days in the calendar year.
- Leap Year Checker — Check if any calendar year is a leap year with 366 days and astronomical explanation.