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Date & time

ISO Week Number Calculator

Find the ISO week number, week-year and Monday-through-Sunday range for a date.

Date conventions explained belowClear start and end values

Set your dates & time

Runs on your device

TIME SUMMARY

Your result

Let’s calculate.

Use the Calculate button to see your result.

Review the method below for assumptions and conventions.

How to use this tool

  1. Enter calendar date.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

The method, explained

ISO weeks start Monday. Week 1 is the week containing January 4, equivalently the year’s first Thursday. The ISO week-year can differ from the calendar year.

A WORKED EXAMPLE

January 1, 2021 belongs to 2020-W53. Its week begins December 28, 2020 and ends January 3, 2021.

What to keep in mind

Uses ISO week-date conventions, not Sunday-start calendars or company fiscal week numbers. Dates near the supported year limits may be rejected when their full week falls outside 0100–9999.

Common questions

Can January dates belong to the previous week-year?

Yes. Days before the first ISO week may belong to the final ISO week of the preceding year.

Does every year have 52 ISO weeks?

No. Some ISO week-years have 53 weeks.

Methodology maintained by ClarityKit. How these tools are built and checked.