ISO Week Number Calculator (ISO-8601 Week and Weekday)
ISO-8601 weeks start on Monday. Week 1 of a year is the week that contains the first Thursday (equivalently, the week that contains 4 January). Early January can belong to the previous ISO year, and late December can belong to the next. This calculator returns the week, the ISO year, and the weekday number 1–7 (Monday–Sunday).
ISO week (YYYY-Www)—
ISO week number—
ISO year—
ISO weekday (1=Mon … 7=Sun)—
Weekday—
How this calculation works
Parse the date as a calendar day in UTC so time zones do not shift the weekday.
ISO weekday: Monday = 1 … Sunday = 7 (JavaScript’s Sunday = 0 is mapped).
Move to the Thursday of that week; that Thursday’s calendar year is the ISO year. Week number is the count of weeks from the ISO year’s 1 January Thursday-aligned start.
Example traps: 1 January 2021 was a Friday in week 53 of ISO year 2020. 29 December 2025 is Monday of 2026-W01.
Worked example
2026-01-01 is Thursday, 2026-W01, weekday 4. 2026-08-26 is Wednesday, 2026-W35, weekday 3. 2021-01-01 is 2020-W53.
Frequently asked questions
Why isn’t 1 January always week 1?
Because week 1 must contain a Thursday. If 1 January is Friday–Sunday, those days often sit in week 52 or 53 of the previous ISO year.
US week numbers?
US calendars often start weeks on Sunday and number week 1 as the week containing 1 January. That is not ISO-8601. This page is ISO only.
What is an ISO year?
The year that “owns” the Thursday of that week. It can differ from the Gregorian calendar year on the date you typed.
Week 53?
Leap weeks happen when the ISO year has 53 Thursdays. 2020 and 2026 have a week 53 in this system for some December/January dates.
Business week vs ISO week?
Many European planners use ISO weeks. US payroll “week 1” may not match. Confirm which standard your software uses.