Week Number Calculator
What week is it? The question sounds trivial until ISO-8601's answer bites: weeks run Monday to Sunday, and week 1 of a year is the one containing the first Thursday — so 1 January can belong to week 52 of the previous year, and some years legitimately have 53 weeks. Pick any date and this calculator returns the ISO week number, the exact Monday-Sunday span it covers, the quarter, and the day-of-year out of 365 or 366.
It defaults to today, which makes it a handy weekly reference: planners, sprint cadences, manufacturing date codes and European calendars all speak ISO weeks. The note explains the first-Thursday rule because that's the part that breaks expectations — a date and its week number can disagree about the year, and the calculator shows which ISO year each week actually belongs to. Your last date is remembered between visits.
How to use
- Enter any date - it starts on today's date for a quick answer.
- Read the ISO week number, the Monday-Sunday span, quarter and day of year.
- Share the week with one tap, or bookmark it - the date is remembered.
Frequently asked questions
What ISO week is it right now?
Open the page and it's calculated instantly from today's date - no input needed. The ISO-8601 system numbers weeks from the week containing the year's first Thursday, with weeks starting Monday, so the number matches what European calendars and most project tools show.
Why can January 1st be in week 52 or 53?
Because week 1 needs the first Thursday. If New Year falls on a Friday, Saturday or Sunday, those days belong to the final week of the old year - and conversely a December 29-31 can already be week 1 of the next. The Monday-Sunday span shown for your date makes the boundary visible.
When does a year have 53 weeks?
When it either starts on a Thursday or is a leap year starting on a Wednesday - 71 of every 400 years qualify, most recently spaced irregularly (2020 had 53 weeks; so will 2026). The day-of-year stat alongside helps you sanity-check where you stand.
Is the ISO week the same everywhere?
No - the US convention often numbers weeks from January 1 regardless of weekdays, and Sunday-start calendars (Middle East, parts of North America) shift every boundary. This calculator follows ISO-8601, the standard used by Europe, most software and international business.