Find the ISO 8601 or US week number for any date, see which dates fall in a given week, or list every week of a year. Fast, free, and built for simple browser-based workflows.
This week
Date → week number
Pick a date to find its week number.
Week → dates
Enter a year and a week number to see its dates.
Every week of a year
Calculated in this browser tab. The dates you pick are not sent anywhere.
Want the weekday instead of the week number? Day of the Week Calculator
Want the full date and time data for a moment? Unix Timestamp Converter
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Week Number Calculator is a free online tool that finds the ISO 8601 or US week number for any date, shows the seven days a given week spans, and lists every week of a year in one table. Pick a date and the week number appears straight away - no manual counting on a calendar.
It comes in handy whenever "week number" means something at work: filing a timesheet marked Week 40, planning a delivery for "KW 42" the way much of Europe does, lining up a school term's week count, or just settling an argument about whether this is week 39 or 40. Sprint planning, payroll cycles and shipping schedules all lean on week numbers more than most people realise.
Everything happens right in your browser, so the date you look up is never sent anywhere or logged. That's one less thing to think about when you're checking a delivery date, a contract deadline, or anything else you'd rather keep off a server you don't control.
Choose ISO 8601 (weeks start Monday - the default) or US (weeks start Sunday) from the selector at the top. It applies to every panel below and is remembered for next time.
Enter a date in Date to week number, or click Today, and the week number, week-year and the week's first and last day show up right away.
Got a week number instead? Enter a Year and Week under Week to dates to see the seven dates it covers.
Enter a Year under Every week of a year to list every week's start and end date, then Copy Table or Download CSV to keep it.
Yes! All our tools are 100% free to use with no hidden costs, no registration required, and no credit card needed.
Absolutely. All tools run entirely in your browser (client-side). Your data never leaves your device and is not sent to our servers.
Yes, once the page is loaded, the tool will continue to work even if you lose internet connection since it runs in your browser.
Yes! All our tools are free for both personal and commercial use.
This page shows it automatically in the "This week" panel at the top, with no need to enter a date - it reads your device's clock and gives both the ISO 8601 week number and the US week number for today.
ISO 8601 weeks start on Monday and week 1 is the week containing the year's first Thursday. US weeks start on Sunday and week 1 is simply the week containing January 1. The same date can land on a different week number depending on which system you use.
Under ISO 8601, a week belongs to whichever year holds its Thursday. If December 29, 30 or 31 falls in a week whose Thursday is already in January, that week - and those December dates - belong to the new year's week 1.
The same Thursday rule runs in reverse: if January 1, 2 or 3 falls in a week whose Thursday is still in December, that week counts as the previous year's week 52 or 53, not week 1 of the new year.
Most years have 52 ISO weeks; a year has 53 only when January 1 falls on a Thursday, or on a Wednesday in a leap year. The tool works this out for whichever year you enter, so you never have to count by hand.
That's the ISO 8601 notation: the year, the week number after "W", and the weekday number after the second dash (1 for Monday through 7 for Sunday). So 2026-W40-6 is the Saturday of week 40, 2026.
Yes. Enter a year under "Every week of a year" and the table fills in with every week's start and end date. Download CSV saves it as a spreadsheet-ready file, or Copy Table copies it as plain text.
No. A week number is worked out from the calendar date alone, so it's the same everywhere in the world. Only the "This week" panel and the Today button, which read your device's current date, depend on your local clock.
Because January 1 falls on a different weekday each year, the Monday that opens ISO week 1 shifts too - anywhere from December 29 of the previous year to January 4. The tool lines this up for you automatically.