Week Number Calculator
Find the week number for today or any date under the ISO-8601 standard, with the week-year, day of year and the exact Monday-to-Sunday span. Reverse mode turns a year and week number back into dates.
Find the current week number.
How to use Week Number Calculator
- Pick any date to see its ISO week number
- Read the week-year, day-of-year, and Monday-to-Sunday range
- Switch to reverse mode to turn a week number into dates
About Week Number Calculator
Week Number Calculator shows the ISO-8601 week number for any date, and it opens on today, so the first thing you see is the current week number — the answer to "what week number is it right now" without touching anything. Pick a different day and it recalculates the ISO week, the ISO week-year, the day-of-year, and the exact Monday-to-Sunday dates that week covers.
Because it follows the real ISO rules, weeks start on Monday and week 1 is the week holding the year's first Thursday, so it handles the awkward late-December and early-January boundaries that trip up simpler calculators. It also shows how many weeks the year holds, 52 or 53, which lets you read today's number as a "week of the year" figure against the full total. A reverse mode goes the other way: type a year and a week number and it returns that week's date range, so you can look up a calendar week by its number instead of by date.
It draws a clear line between ISO figures and ordinary ones — the day-of-year count and the calendar year sit next to the ISO week and week-year, and a note appears whenever those two disagree. Everything is computed on the plain calendar date you pick, so your time zone never shifts the result. It is completely free, needs no sign-up, and runs entirely in your browser, so nothing you enter is ever uploaded.
Frequently asked questions
- What week number is it right now?
- Open the tool and it starts on today's date, so the current ISO week number is already shown at the top with that week's Monday-to-Sunday range. You don't have to pick anything — the answer loads immediately, and it updates the moment you choose a different day.
- What week of the year is it?
- The forward mode shows today's ISO week alongside the total number of weeks the year holds, so you can read it as "week 37 of 52" and see how far through the year you are. Both the week number and the yearly total sit in the stat grid together.
- Does this use ISO week numbers or US week numbering?
- ISO-8601 only. Weeks run Monday to Sunday and week 1 is the week containing the year's first Thursday — the same as the week containing 4 January. The US and Canadian convention, where weeks start on Sunday and week 1 is whichever week holds 1 January, is not offered, and the two systems often differ by one.
- Why does the ISO week-year sometimes differ from the calendar year?
- Because an ISO week is never split between two years, the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next. The tool shows the ISO week-year as its own stat and prints a note whenever it does not match the calendar year.
- Which years have 53 ISO weeks instead of 52?
- Years where 1 January falls on a Thursday, plus leap years that start on a Wednesday. The stat grid shows the total for whichever year you are looking at, and the reverse mode rejects any week number above it with a message naming the real maximum.
- How do I turn a week number back into dates?
- Switch to "Week number → dates" and enter the ISO week-year and a week number to look up a calendar week by its number. You get that week's Monday and Sunday written out in full, the same two dates in YYYY-MM-DD form, and the number of weeks the year holds.
- Is the day-of-year figure ISO-based too?
- No. Day of year counts from 1 January of the ordinary calendar year, so 1 January is day 1 and the total is 365 or 366 depending on whether that year is a leap year. Only the week number and week-year follow the ISO rules.
- Does my time zone change the week number?
- No. The date you pick is read as a plain calendar date — year, month and day — and all the ISO arithmetic is done on those three numbers, so the same date returns the same week anywhere in the world. Your device's clock only decides which date the picker starts on.
- What does Copy summary put on the clipboard?
- A single line combining the week and its date range, in the form "ISO week 12, 2026 (2026-03-16 to 2026-03-22)".
- What dates and years does it accept?
- The forward mode uses your browser's date field, and impossible dates such as 30 February are refused with "Please enter a valid date" rather than silently rolling into March. The reverse mode accepts week-years from 1 to 9999.