ISO Week Number Calculator
Type any calendar date and this tool returns its ISO 8601 week number, the ISO week-numbering year, and the weekday counted from Monday as 1. It also runs in reverse, turning an ISO year, week, and weekday back into an exact Gregorian date. ISO week 1 is defined as the week that contains the first Thursday of the year, equivalently the week that holds 4 January.
How to find the ISO week number
- In the 'Date to ISO week' panel, type the Year, Month, and Day.
- Read the large result, formatted as ISO year and week, for example 2025-W01.
- Check the weekday line below it, where Monday is 1 and Sunday is 7.
- Click Copy to grab the full sentence such as 2024-12-30 is ISO 2025-W01-1 (Monday).
- To go the other way, fill the 'ISO week to date' panel with an ISO year, a Week from 1 to 53, and a Weekday from the dropdown.
- Read the Gregorian date it returns and copy it if needed.
Examples
A late December date that belongs to the next ISO year
Year 2024, Month 12, Day 30
ISO year 2025, week 1, weekday 1 (Monday).
A New Year date that belongs to the previous ISO year
Year 2027, Month 1, Day 1
ISO year 2026, week 53, weekday 5 (Friday).
Reverse: turn an ISO week back into a date
ISO year 2026, week 53, weekday 5
2027-01-01
Frequently asked questions
What is an ISO week number?
ISO 8601 divides the year into numbered weeks that always run Monday to Sunday. Week 1 is the week containing the first Thursday of the year, which is the same as the week that contains 4 January. This tool applies that rule exactly rather than the looser 'first week with any days in January' conventions used by some spreadsheets.
Why can the ISO year differ from the calendar year?
A week is assigned to whichever year holds its Thursday. So a date in late December can sit in a week whose Thursday falls in January, pushing it into the next ISO year, and an early January date can belong to the previous ISO year. For instance 30 December 2024 is ISO 2025-W01, and 1 January 2027 is ISO 2026-W53.
How many ISO weeks are in a year?
Most ISO years have 52 weeks. A year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. That extra week is why the Week field in the reverse panel accepts values up to 53.
Which day is the first day of the ISO week?
Monday. ISO 8601 numbers weekdays Monday as 1 through Sunday as 7, which differs from the common Sunday-first calendars used in North America. The reverse panel's Weekday dropdown lists each number next to its day name so there is no ambiguity.
Can I convert a week number back to a date?
Yes. The lower panel takes an ISO year, a week number, and a weekday and returns the matching calendar date. This is handy when a schedule refers to week numbers and you need the actual Monday or Friday of that week.
What date range does it handle?
The math uses a proleptic Gregorian day count, so it works for dates far in the past and future without relying on the browser's Date object. Year, month, and day must be whole numbers, month between 1 and 12, and day between 1 and 31.
Does it account for leap years?
Yes. The underlying day-number conversion follows the full Gregorian leap year rule, including the century and 400-year exceptions, so 29 February and the week counts around it come out correct.
Is my data sent anywhere?
No. The calculation runs entirely in your browser with pure functions, so the dates you enter never leave your device. There is also a public JSON API for the same engine if you want to script the conversion.
How is this different from a plain week-of-year count?
A naive count numbers weeks from 1 January regardless of weekday, which splits a week across two numbers and drifts every year. ISO weeks are anchored to Thursdays so each week has a single stable number that agencies, payroll systems, and manufacturing schedules can all agree on.
Learn more
- ISO 8601 week numbering explained
How ISO 8601 defines week 1, why the ISO year can drift from the calendar year, and when a year gets a 53rd week.
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