Date & Time
Week Number Calculator
Week Number — 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. This page runs that same calculation on your numbers, in your browser: no signup, no uploads, nothing stored.
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 returns the ISO-8601 week number (weeks start on Monday; week 1 holds the year's first Thursday) along with the day-of-year. That is the whole of the calculation — the mistakes happen in the inputs. It matters whenever spans between dates, times or zones drive a plan, a deadline or a record. Worked example: 1 January 2025 falls in ISO week 1 ; the ISO week containing the first Thursday of the year is week 1. Use the calculator with your own figures; it runs 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
Use it when reading a fiscal-week calendar
Use it for quick week-number lookup
Use it when finding the day-of-year
Use it when coordinating teams by week
Use it when working with ISO week numbers
Use it 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.