Date & Time
Week Number Calculator
Week Number Calculator is a browser-based calculator tool on CalcToolsBase. Free, browser-based Week Number Calculator with instant, clearly-labelled results. No signup, no uploads. To use it, enter your values and view the result directly in your browser — no signup and nothing to install.
Week Number Calculator lets you find the ISO week number for a date in seconds: enter your date for the week number and the day-of-year, all worked out locally in your browser.
About this calculator
Week Number Calculator is built to find the ISO week number for a date without a server round-trip. Provide your date and it uses the ISO-8601 week rule to produce the week number and the day-of-year, instantly and entirely in your browser, which makes it handy for quick checks and repeat runs.
Made for planners and teams working by week, it gives the week number and the day-of-year from your date in one step — clearly laid out, free to use, and computed entirely on your device.
How to use
- Enter a date.
- Press Calculate to find the week number.
- Read the ISO week number and its week-year.
- Check the weekday and day-of-year for that date.
Why use Week Number Calculator
Fast
Calculations run in your browser. No round trip to a server.
Private
Your inputs never leave your device. Nothing is uploaded.
Free
No signup, no paywall, no ads-in-results, no watermarks.
Accurate
Uses standard calendar arithmetic where applicable. Check critical schedules, payroll, and official holiday dates before relying on results.
Mobile-ready
Optimised for phones, tablets, and desktops alike.
Transparent
Honest disclaimers wherever browser limits or regional rules apply.
Common uses
Week Number when reading a fiscal-week calendar
Week Number for quick week-number lookup
Week Number when finding the day-of-year
Week Number when coordinating teams by week
Week Number when working with ISO week numbers
Week Number when a system uses week numbers
Technical notes
Week Number Calculator returns the ISO-8601 week number (weeks start on Monday; week 1 holds the year's first Thursday) along with the day-of-year.
Calculations use the calendar only and do not adjust for time-zone shifts unless a field asks for a zone.
Because it runs locally, results appear instantly with no server round-trip.
FAQ
What is an ISO week-year?
Under ISO 8601 the first week of a year is the one containing its first Thursday, so an early-January date can belong to the previous year's final week — 2021-01-01 is week 53 of 2020.
How are weeks numbered?
Weeks run Monday to Sunday and are numbered 1 to 52 or 53 according to ISO 8601.
What else does it show?
Besides the ISO week and week-year, it shows the date's weekday and its day-of-year number.
Does the Week Number Calculator send my data to a server?
No. The Week Number Calculator runs entirely in your browser — your inputs never leave your device, and nothing is uploaded or stored.
Worked example
1 January 2025 falls in ISO week 1; the ISO week containing the first Thursday of the year is week 1.