ISO Week Number Calculator
Find the ISO 8601 week number and ISO year for any date.
Quick answer: ISO Week Number Calculator: find the ISO 8601 week number and ISO year for any date. Runs entirely in your browser, free, no signup.
Last updated
Good to know
ISO 8601 defines weeks with two firm rules that catch people out: weeks start on Monday, and week 1 is the week containing the first Thursday of the year (equivalently, the week holding January 4). That's why the first few days of January can belong to week 52 or 53 of the previous ISO year, and the last days of December can already fall in week 1 of the next one.
This split between calendar year and ISO week-numbering year is the single most common source of confusion. December 31 can legitimately report as "week 1" of the following year — that's correct ISO behavior, not a bug. When you cite a week number in a report, cite the ISO year with it, or a reader may look at the wrong calendar row.
ISO week numbering is the European and international standard, and it's what Outlook and Google Calendar show by default in most non-US locales — so a week number generated here will match what colleagues on those tools see. It is deliberately not the North American convention, where weeks often start on Sunday and week 1 is simply the one containing January 1.
For the day side of the same calendar questions — which weekday a given date falls on — pair this with the Day of Week Finder.
Frequently asked questions
- Is the week number calculator free?
- Yes — free with no signup.
- Why is the ISO year sometimes different from the calendar year?
- Days early in January can belong to week 52/53 of the previous ISO year, and vice versa for late December.
- Does it match what Outlook and Google Calendar show?
- Yes — both default to ISO 8601 week numbering in EU locales.
- Are my dates uploaded?
- No — every calculation runs in your browser.
- What about US-style week numbering?
- US numbering starts weeks on Sunday and uses different rules. This tool is ISO-only.