How to use ISO Week Number Calculator

Leave Lookup on Find ISO week for a date.
Enter a Calendar date, for example 2025-12-31.
The ISO week updates as you change the date.
Read the ISO week code, the ISO weekday and the Monday to Sunday range.
Check Weeks in ISO year for the 52 or 53 week count.

Example: ISO Week Number Calculator

Look up the ISO week for the first day of 2021.

You add

Lookup: Find ISO week for a date. Calendar date: 2021-01-01.

You get

The result is 2020-W53-5. That ISO week runs Monday 2020-12-28 through Sunday 2021-01-03, so the calendar year and the ISO week-year differ by one.

Options

Lookup: Reverses the direction. Find dates for an ISO week takes a week-year and week number instead of a date.
Calendar date (for date lookup): Uses YYYY-MM-DD with a picker. Dates outside 1900 to 2100, or dates that do not exist such as 30 February, are refused rather than rolled forward.
ISO week-year and ISO week number: Used only in week mode. A pair that does not exist, such as week 53 in a 52-week year, is refused and the message gives the valid range.

Supported inputs and limits

Calendar dates from 1900 through 2100 are supported, and a week you look up must fall inside that range. Week 53 is accepted only in an ISO year that has it. The tool compares calendar dates, so your time zone and daylight saving never change the result. Week lookup returns the Monday and Sunday plus the week count, not each individual day.

Where your input is processed

This tool processes your input in this browser. Your text and files are not uploaded to UseFreeTools. Check this tool's limits for anything it may save on your device.

Week-year is its own field

ISO 8601 numbers weeks from Monday and puts week 1 in the week containing 4 January. Because 1 January can fall late in a week, a date at either end of the New Year can belong to a different week-year than the calendar year printed on it.

Python documentation: date.isocalendar and the ISO week rule

Where 52 and 53 week years come from

A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 begins on a Thursday, so ISO 2026 has week 53.

ISO 8601-1:2019, Date and time: week numbering

Questions about ISO Week Number Calculator

Why is 31 December 2025 in week 1 of 2026?

ISO weeks start on Monday and week 1 contains 4 January. The week that contains 4 January 2026 starts on Monday 29 December 2025, so 31 December 2025 is ISO 2026-W01 weekday 3, a Wednesday.

How do I turn a week number back into dates?

Set Lookup to Find dates for an ISO week, enter the ISO week-year and week number, and read the Monday and Sunday.

Which years have 53 ISO weeks?

A long ISO year has 53 weeks. In the supported range 2026 has 53 while 2025 has 52. The Weeks in ISO year row shows the count for the week-year you are looking at.

Does the tool use my local time zone?

No. It works on calendar dates only, so the same date gives the same week number wherever you are.

Project manager: Tony Hines · Content updated 29 September 2026 · Report a problem

ISO Week Number Calculator

Find the ISO week controls