Week Number Calculator
Find the ISO week number for any date
Week Number Calculator is a free online tool from BrowserUtils that find the iso week number for any date. It runs entirely in your browser — your data never leaves your device. No account required.
How to use Week Number Calculator
- 1 Paste or type your input into the editor above.
- 2 The tool processes your data instantly — right in your browser, with nothing sent to a server.
- 3 Copy the result with one click or continue editing your input.
About Week Number Calculator
Free online ISO week number calculator. Find the week number for any date according to the ISO 8601 standard. This tool runs entirely in your browser — your data is never sent to a server. Just paste your input, get instant results, and copy with one click. No sign-up or installation required.
Week Number Calculator specs
- Runtime
- 100% client-side (browser)
- Built on
- Native Date and Intl.DateTimeFormat APIs with full IANA timezone database support
- Cost
- Free — no account, no rate limits, no usage caps
- Browser support
- Chrome 90+, Firefox 88+, Safari 14+, Edge 90+
- Part of
- 299 developer tools on BrowserUtils (100% client-side)
Questions
How is the ISO week number determined?
ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday.
Can the ISO week year differ from the calendar year?
Yes, dates in early January may belong to the last ISO week of the previous year, and late December dates may belong to week 1 of the next year.
What is the maximum number of ISO weeks in a year?
Most years have 52 ISO weeks, but some years (called "long years") have 53. This happens when January 1 falls on a Thursday, or on a Wednesday in a leap year.
How do I find the week number for today?
Open the tool and it defaults to the current date with the ISO week number already displayed. You can change the date to look up any other week number.
Does the week number calculator work offline?
Yes. The ISO week calculation runs entirely in your browser using JavaScript. No internet connection is needed after the page loads.
Comments
Related tools
More Date & Time
Date Difference CalculatorDate Add/SubtractTimezone ConverterWorld ClockISO 8601 ConverterAge CalculatorCountdown TimerStopwatch
View all Date & Time tools
Comments