Or type a date: YYYY-MM-DD
QUICK:
Ready to calculate
Enter a date and click Calculate Week// find iso week number for any date in one click
Find the ISO 8601 week number and ISO year for any calendar date. Instantly calculate week boundaries, day of week, and quarter from any date input.
Ready to calculate
Enter a date and click Calculate WeekUse the date picker or type any date in the text field.
Get the ISO week number, ISO year, week boundaries, and day of week instantly.
Click "Show Full Year" to see every ISO week in that calendar year.
The ISO week number follows the ISO 8601 standard: weeks start on Monday, and week 1 is the week containing the year's first Thursday. The ISO year may differ from the calendar year near year-end boundaries.
An ISO week number (defined in ISO 8601) is a way to number the weeks of a year. Week 1 is the week that contains the year's first Thursday. Weeks always run Monday to Sunday, so a year has either 52 or 53 ISO weeks.
Near year boundaries, a Monday–Sunday week can straddle two calendar years. ISO 8601 assigns the entire week to whichever year contains its Thursday. So January 1st could belong to the previous ISO year, or December 31st could belong to the next ISO year.
Most ISO years have 52 weeks. A "long year" (with 53 weeks) occurs when January 1st is a Thursday, or — in a leap year — when January 1st is a Wednesday or Thursday. This happens about 71 times per 400-year cycle.
The ISO 8601 week date format is YYYY-Www-D, for example 2024-W12-3 means ISO year 2024, week 12, Wednesday (day 3). This calculator shows all components of that format.
Not exactly. Most of Europe and many international standards use ISO 8601 (Monday start, first Thursday = week 1). The US and some other countries count weeks differently — often starting on Sunday with different week-1 rules. This tool strictly follows ISO 8601.
In PHP use date('W') and date('o') for ISO year. In JavaScript, there is no built-in, but you can compute it by finding the nearest Thursday. In Python, use datetime.isocalendar(). In SQL (MySQL) use WEEK(date, 3) or YEARWEEK(date, 3).
The ISO week number is a standardized way of numbering calendar weeks, defined by the international standard ISO 8601. Unlike the intuitive assumption that week 1 simply starts on January 1st, the ISO standard has a precise rule: Week 1 is the week that contains the first Thursday of the year. All weeks begin on Monday and end on Sunday.
This seemingly simple rule has an important consequence: the ISO year (sometimes called the "week year") can differ from the Gregorian calendar year by one day at most. If January 1st falls on a Friday, Saturday, or Sunday, it belongs to the last ISO week of the previous year, because the majority of that week (Monday through Wednesday or Thursday) is in December.
💡 Looking for premium web development assets? MonsterONE offers unlimited downloads of templates, UI kits, and developer tools — worth checking out.
To understand ISO week numbering, remember these key rules:
The practical difference between ISO week numbers and simple calendar week numbers becomes visible at year boundaries. Consider December 29, 2025 — it's a Monday, the start of a week whose Thursday (January 1, 2026) falls in 2026. Therefore, ISO 8601 assigns December 29–January 4 to Week 1 of ISO year 2026, even though most of those days are in 2025.
Conversely, some January dates get assigned to the previous ISO year. January 1, 2016 was a Friday — its week's Thursday was December 31, 2015, so that week belongs to ISO year 2015, Week 53.
Different programming languages offer varying levels of built-in support for ISO week numbers:
date('W') returns the ISO week number, and date('o') returns the ISO year. These are directly ISO 8601 compliant.datetime.isocalendar() method returns a named tuple with year, week, and weekday — the cleanest built-in solution.getISOWeek()) or dayjs (.isoWeek()).WEEK(date, 3) or YEARWEEK(date, 3) where mode 3 is ISO 8601 compliant. In PostgreSQL, use EXTRACT(WEEK FROM date) which also follows ISO 8601.ISOWEEKNUM() function (available since Excel 2013) returns the ISO week number directly.ISO week numbers are widely used in business and engineering contexts where consistent, unambiguous week identification is critical:
The ISO year (week year) is the year to which the ISO week belongs. It is written as a four-digit number, just like the Gregorian year, but it only matches the Gregorian year for weeks entirely within that calendar year. The notation YYYY-Www (e.g., 2024-W34) makes this explicit: YYYY is the ISO year, and ww is the zero-padded week number.
The full ISO 8601 week date format includes the day of the week: YYYY-Www-D, where D is 1 for Monday through 7 for Sunday. For example, 2024-W34-5 is the Friday of week 34 in ISO year 2024.
To find the ISO week number for any date without a calculator, follow these steps:
This algorithm makes it clear why the ISO year can differ from the calendar year: it's determined entirely by where the Thursday of your week falls, not by where Monday falls.