Week Number Calculator

1. Date to Week mode: select any date using the calendar picker. The calculator returns the ISO week number, the Monday start date, the Sunday end date, and which day of the week the selected date falls on.
2. Week to Date mode: enter a year and an ISO week number to look up the exact Monday-to-Sunday date range for that week. Useful for scheduling, reporting periods, and payroll cycles that use week numbers.

This calculator follows the ISO 8601 standard: weeks start on Monday, and Week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4).

ISO 8601 Week 1 = week containing the first Thursday of the year
Equivalent rule: week containing January 4

Week number of any date:
1. Find the nearest Thursday to the date
2. That Thursday's year is the ISO year
3. Count weeks from week 1 of that year

Example: January 1, 2026 is a Thursday
It falls in Week 1 of 2026

Example: December 29, 2025 is a Monday
Its nearest Thursday is January 1, 2026
So December 29, 2025 = Week 1 of 2026 (not 2025!)

Week 1 start dates for recent years: