What week-number convention does this calculator use?
It uses ISO 8601 week dates: weeks start on Monday and week 1 is the week containing January 4.
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Why can the ISO week-year differ from the calendar year?
Dates near New Year can belong to the final ISO week of the previous week-year or the first ISO week of the next week-year.
What dates are shown for the week?
The result includes the Monday start and Sunday end of the selected ISO week.
Is this the same as every U.S. calendar week number?
No. Some calendars use Sunday starts or define week 1 differently. This page explicitly uses ISO week numbering.
Worked example: Week Number
Enter the following values in the Week Number Calculator and calculate to reproduce this result. Dates below use year-month-day order.
- Calendar date: 2026-01-01
Example result: Week 1. January 1, 2026 belongs to ISO week 1 of 2026.
Calculation used: ISO week starts Monday; week 1 contains January 4
How to interpret this example
ISO weeks start on Monday, and week 1 contains January 4. The ISO week-year can differ from the calendar year near New Year. Read both the week number and the associated ISO year when labeling a schedule.
Frequently asked questions
Sources and methodology
This calculator uses transparent browser-side date/time arithmetic. The references below provide standards or official context relevant to the calculation.