Week number calculator

How the week number calculator works

The calculator follows ISO 8601, the international standard for week numbers. It takes the date you enter as a plain calendar date, so the answer is the same in every time zone. Three steps give the result:

1. Find the Monday. ISO weeks run Monday to Sunday, so the date is placed in its Monday–Sunday week.
2. Look at the Thursday. The Thursday of that week decides the ISO year. A week belongs to the year that contains most of its days, and the Thursday is always in that year.
3. Count from week 1. Week 1 is the week that contains January 4 (equivalently, the first Thursday). The number of weeks from that Monday, plus one, is the week number.

The result is shown as an ISO code such as 2026-W40: the ISO year, a "W" and a two-digit week. The code is unambiguous, which matters because the ISO year and the calendar year differ for a few days around New Year.

New Year examples

These dates show why the ISO year matters. December dates can be in week 1 of the next year, and early January dates can be in week 52 or 53 of the previous year. The US column uses Sunday-start weeks where week 1 contains January 1, the default in many US calendars and in Excel's WEEKNUM.

Using the result

Copy the result to paste the ISO code and the Monday–Sunday range into a plan, ticket or spreadsheet, or share a link to the calculator. For a whole year at a glance, open the 2026 week-number table; to go from a week number back to dates, use the week to date converter. Each week also has its own page with all seven dates, US week numbers and holidays.