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.

Enter your values and tap Calculate.

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

  1. Enter a date.
  2. Press Calculate to find the week number.
  3. Read the ISO week number and its week-year.
  4. 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.