DevToolKit

ISO Week Converter

Convert dates to ISO 8601 week numbers and vice versa. Supports US/Middle East systems, fiscal year calculations, and date range expansion. 100% private.

Current ISO Week
W122026
Year Progress22.1%
Day 81 / 365Quarter 1

Global Settings

Result Details

Start of Week
Monday, March 16, 2026
End of Week
Sunday, March 22, 2026
Week Number
W12
Reference Year
2026

ISO 8601 rules: Week 1 is the week with the first Thursday. Weeks start on Monday. Dec 29-31 can be Week 1 of next year.

Developer Snippets

JavaScript (date-fns)
// Using date-fns
import { getISOWeek } from 'date-fns';
const week = getISOWeek(new Date());
Python
from datetime import date
week = date.today().isocalendar()[1]
PHP
<?php
echo date('W'); // ISO-8601 week number ?>

First Thursday Rule

ISO 8601 week 1 is the week that contains the first Thursday of the year. This ensures week 1 always has at least 4 days in the new year.

Year Boundary

Did you know? In the ISO system, some years have 53 weeks. This happens every 5-6 years to prevent calendar drift.
Was this tool helpful?

How to Use

The ISO Week Converter is a professional dashboard for network administrators, developers, and project managers to handle complex date calculations across different numbering systems.

How to use this tool

  1. Select Numbering System: Choose between ISO 8601 (Global standard, Mon-Sun), US System (Sun-Sat), or Fiscal Year mode.
  2. Pick a Date: Use the interactive date picker to see the corresponding week number, day of the year, and quarter.
  3. Explore Calendar: Switch to the "Calendar View" tab to see a full month layout with week numbers clearly labeled in the side gutter.
  4. Year Progress: Monitor the visual progress bar to see exactly how much of the current year or fiscal period has passed.
  5. Integrate: Copy the provided code snippets for JavaScript, Python, or PHP to implement these calculations in your own applications.

About This Tool

Understanding ISO 8601 Weeks

The ISO 8601 week date system is part of the ISO 8601 date and time standard. The system is used mainly in government and business for fiscal years, as well as in timekeeping. The system uses the same year-week-day notation as the standard Gregorian calendar.

A key rule of the ISO system is that Week 01 is the week with the year's first Thursday. This means that if January 1st is a Friday, it actually belongs to the last week of the previous year.

Why Use This Tool

Why DevToolkit's Converter?

Most online week converters only show the current week. Our tool provides a full workbench including **fiscal year adjustments**, **US vs ISO switching**, and a **visual calendar gutter**. All calculations are done locally in your browser using the reliable `date-fns` library, ensuring 100% privacy for your planning and data.

FAQ

What is an ISO 8601 week?
ISO 8601 defines weeks as Monday-to-Sunday periods. Week 1 of a year is the week containing the first Thursday of the year. This means January 1st can fall in week 52 or 53 of the previous year.
What week numbering systems are supported?
It supports the ISO system (Monday-Sunday, used internationally), the US system (Sunday-Saturday), and Middle East systems (Saturday-Friday). It also includes customizable fiscal year calculations.
Can I convert a range of weeks to dates?
Yes. The tool can expand a range of ISO weeks into all the individual dates they contain, which is useful for project planning and scheduling.
Is my data private?
Yes. All calculations are performed locally in your browser. No dates or data are sent to any server.
NavigateOpenEscClose