Week Number Calculator
Find the ISO 8601 week number (1–53) and week-year for any date.
ISO week
What is Week Number Calculator?
The Week Number Calculator determines the official ISO 8601 week number and corresponding week-year for any date. Under ISO 8601, Monday is considered the first day of the week, and Week 1 is defined as the week containing the first Thursday of the Gregorian year.
How to use Week Number Calculator
- 1Select any date using the date picker or enter a date in YYYY-MM-DD format.
- 2Review the computed ISO week number (e.g., W42) and corresponding ISO week-year.
- 3Use the output for project scheduling, sprint planning, or payroll compliance.
Key features
ISO 8601 Standard Compliance
Calculates official ISO week numbers where Monday is day 1 and Week 1 contains the first Thursday of the year.
ISO Week-Year Output
Displays the correct ISO week-year, accounting for year boundaries where days in early January or late December belong to adjacent week-years.
Instant In-Browser Calculation
Calculates immediately on date selection without network requests or external dependencies.
Examples & use cases
Standard Mid-Year Date
Wednesday in mid-October 2026
2026-10-14
Week number: W42, ISO week-year: 2026
Year Boundary Week 1
New Year Day falling on Thursday
2026-01-01
Week number: W1, ISO week-year: 2026
Sunday Belonging to Prior Year Week
January 1 falling on Sunday
2023-01-01
Week number: W52, ISO week-year: 2022
Frequently asked questions
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