Week Number Calculator

Find the ISO 8601 week number for any date, and look up which dates fall in a given week. Pick a date (or use today) to see its ISO week number, the week-numbering year, the week's Monday start and Sunday end dates, the weekday, the day-of-year, how many weeks the year has, and how many weeks are left in the year. Reverse it too: enter a year and week number to get that week's date range. Everything runs in your browser.

Look up a week number

Enter an ISO week-numbering year and a week number to see the date range that week covers.

ToolsSoup's Week Number Calculator is a free online tool for working with ISO 8601 week numbers. Choose any date — or load today — and instantly see its ISO week number, the week-numbering year, the Monday-to-Sunday dates that week spans, the weekday, the day-of-year, the total weeks in that year, and how many weeks are left. Need the reverse? Type a year and a week number and it returns the exact date range. Everything runs locally in your browser, so nothing you enter is uploaded.

What is an ISO 8601 week number?

Under the ISO 8601 standard, each calendar week is numbered from 1 to 52 or 53. Weeks always start on Monday and end on Sunday, and week 1 is defined as the week containing the year's first Thursday — equivalently, the week containing January 4. Because of that rule, the first days of January can belong to the last week of the previous year, and the final days of December can belong to week 1 of the next year. That is why ISO also tracks a separate week-numbering year, which can differ from the calendar year for a few days at each boundary.

How to find the week number for a date

The calculator updates the moment you choose a date — there is nothing to install:

  1. Pick a date in the date field, or click Today to load the current date.
  2. Read the badge to see the ISO week number and the week-numbering year.
  3. Review the details — the Monday start date, the Sunday end date, the day-of-year, the weeks in the year, and how many weeks remain.
  4. Click Copy to grab the result, or Clear to start over.

How to convert a week number back to dates

Use the reverse lookup at the bottom to turn a week number into a calendar range:

  1. Enter the ISO week-numbering year, for example 2026.
  2. Enter the week number, for example 25.
  3. Click Find dates to see the Monday start and Sunday end of that week.

Why use this week number calculator?

  • Returns the correct ISO 8601 week number for any date, with weeks starting on Monday.
  • Shows the week-numbering year, which can differ from the calendar year near January 1 and December 31.
  • Gives the full Monday-to-Sunday date range for the week, not just the number.
  • Tells you how many ISO weeks the year has (52 or 53) and how many weeks are left.
  • Includes a reverse lookup that turns a year and week number back into a date range.
  • Runs entirely in your browser — nothing you enter is uploaded or stored.

Frequently asked questions

What week number is it today?

Click the Today button to load the current date and the calculator shows this week's ISO 8601 week number right away, along with the Monday-to-Sunday dates it covers.

Which day does an ISO week start on?

ISO 8601 weeks always start on Monday and end on Sunday. This is different from the US convention, where calendars often start the week on Sunday.

How is week 1 decided?

Week 1 is the week that contains the year's first Thursday, which is the same as the week containing January 4. A simpler way to think about it: week 1 is the first week with the majority of its days in the new year.

Why does the week-numbering year sometimes differ from the calendar year?

Because weeks do not split across the year boundary, a date like December 31 can fall in week 1 of the next year, and January 1 can fall in the last week of the previous year. The week-numbering year follows the week, so for those few days it differs from the calendar year shown on the date.

Can a year have 53 weeks?

Yes. Most years have 52 ISO weeks, but a year has 53 weeks when it starts on a Thursday, or starts on a Wednesday and is a leap year. The calculator reports the exact number for whichever year you enter.

Is my data kept private?

Yes. Every calculation runs locally in your browser with JavaScript. The dates and week numbers you enter are never sent to or stored on any server.