ISO Week to Date Converter
Convert ISO 8601 week indexes to calendar dates and back for free. Discover year start offsets, total week counts, and browse the complete annual calendar directory.
Full list of ISO weeks for 2026. Click any week to load it into the converter.
| ISO Week | Monday (Start Date) | Sunday (End Date) | Action |
|---|---|---|---|
| W01 | Dec 29, 2025 | Jan 4, 2026 | |
| W02 | Jan 5, 2026 | Jan 11, 2026 | |
| W03 | Jan 12, 2026 | Jan 18, 2026 | |
| W04 | Jan 19, 2026 | Jan 25, 2026 | |
| W05 | Jan 26, 2026 | Feb 1, 2026 | |
| W06 | Feb 2, 2026 | Feb 8, 2026 | |
| W07 | Feb 9, 2026 | Feb 15, 2026 | |
| W08 | Feb 16, 2026 | Feb 22, 2026 | |
| W09 | Feb 23, 2026 | Mar 1, 2026 | |
| W10 | Mar 2, 2026 | Mar 8, 2026 | |
| W11 | Mar 9, 2026 | Mar 15, 2026 | |
| W12 | Mar 16, 2026 | Mar 22, 2026 | |
| W13 | Mar 23, 2026 | Mar 29, 2026 | |
| W14 | Mar 30, 2026 | Apr 5, 2026 | |
| W15 | Apr 6, 2026 | Apr 12, 2026 | |
| W16 | Apr 13, 2026 | Apr 19, 2026 | |
| W17 | Apr 20, 2026 | Apr 26, 2026 | |
| W18 | Apr 27, 2026 | May 3, 2026 | |
| W19 | May 4, 2026 | May 10, 2026 | |
| W20 | May 11, 2026 | May 17, 2026 | |
| W21 | May 18, 2026 | May 24, 2026 | |
| W22 | May 25, 2026 | May 31, 2026 | |
| W23 | Jun 1, 2026 | Jun 7, 2026 | |
| W24 | Jun 8, 2026 | Jun 14, 2026 | |
| W25 | Jun 15, 2026 | Jun 21, 2026 | |
| W26 | Jun 22, 2026 | Jun 28, 2026 | |
| W27 | Jun 29, 2026 | Jul 5, 2026 | |
| W28 | Jul 6, 2026 | Jul 12, 2026 | |
| W29 | Jul 13, 2026 | Jul 19, 2026 | |
| W30 | Jul 20, 2026 | Jul 26, 2026 | |
| W31 | Jul 27, 2026 | Aug 2, 2026 | |
| W32 | Aug 3, 2026 | Aug 9, 2026 | |
| W33 | Aug 10, 2026 | Aug 16, 2026 | |
| W34 | Aug 17, 2026 | Aug 23, 2026 | |
| W35 | Aug 24, 2026 | Aug 30, 2026 | |
| W36 | Aug 31, 2026 | Sep 6, 2026 | |
| W37 | Sep 7, 2026 | Sep 13, 2026 | |
| W38 | Sep 14, 2026 | Sep 20, 2026 | |
| W39 | Sep 21, 2026 | Sep 27, 2026 | |
| W40 | Sep 28, 2026 | Oct 4, 2026 | |
| W41 | Oct 5, 2026 | Oct 11, 2026 | |
| W42 | Oct 12, 2026 | Oct 18, 2026 | |
| W43 | Oct 19, 2026 | Oct 25, 2026 | |
| W44 | Oct 26, 2026 | Nov 1, 2026 | |
| W45 | Nov 2, 2026 | Nov 8, 2026 | |
| W46 | Nov 9, 2026 | Nov 15, 2026 | |
| W47 | Nov 16, 2026 | Nov 22, 2026 | |
| W48 | Nov 23, 2026 | Nov 29, 2026 | |
| W49 | Nov 30, 2026 | Dec 6, 2026 | |
| W50 | Dec 7, 2026 | Dec 13, 2026 | |
| W51 | Dec 14, 2026 | Dec 20, 2026 | |
| W52 | Dec 21, 2026 | Dec 27, 2026 | |
| W53 | Dec 28, 2026 | Jan 3, 2027 |
Most ISO calendar years have exactly 52 weeks. The following years contain a "leap week" (53 weeks) due to their starting day.
Why Use Our ISO Week to Date Converter?
Bidirectional Conversions
Translate ISO 8601 week codes and years into Gregorian date ranges, or pick a calendar date to inspect its week index and day numbering code instantly.
53-Week Smart Validation
Prevents invalid date requests. The tool dynamically identifies if a year has 52 or 53 weeks, preventing week-overflow entries and flashing clear warnings.
Full Annual Directory index
Explore the complete calendar breakdown. List all weeks for any selected year with date intervals, and highlight the current calendar week dynamically.
100% Secure & Client-Side
No calendar queries or dates are sent to remote servers. All ISO calculations run locally inside your browser, safeguarding your complete privacy.
Common Use Cases for ISO Week to Date Converter
Financial & Fiscal Audits
Reconcile transaction periods in corporate reporting. Many financial statements index earnings by week (e.g. W12). Quickly verify which Gregorian dates align with audit records.
Project Management & Planning
Coordinate sprint schedules and team goals. Calculate exactly when a week begins and ends to set precise project start dates and milestones.
Supply Chain & Logistics
Interpret delivery sheets and manufacturer schedules that often label shipping timelines by ISO week index (e.g. W44 delivery window).
ERP & Business Systems
Configure Enterprise Resource Planning (ERP) data nodes that store transactional entries using ISO-8601 week formatting specifications.
Software Development & API Testing
Verify ISO week string parsers during database schema developments or unit testing of date libraries inside web applications.
Academic & School Schedules
Map school terms, course calendars, and syllabus lists that are commonly structured on week-by-week intervals throughout the academic year.
Understanding the ISO 8601 Week System
What is the ISO 8601 Week Date Standard?
ISO 8601 is an international standard covering the exchange of date and time-related data. The ISO week-numbering system is a leak-proof calendar system where years are divided into complete weekly blocks. Each week starts on a Monday and is numbered sequentially from 1 to 52 or 53. Unlike the standard Gregorian calendar, there are no partial weeks; every week has exactly 7 days, and every year starts on a Monday. This structure is highly valued in business planning, retail supply chains, fiscal budgeting, and international logistics because it allows clean weekly comparison charts without standard month-end date offsets.
How to Convert Dates and ISO Weeks
- 1. Week to Date Range Conversion: Input your target ISO year and choose a week number (1–53). The calculator will dynamically compute the exact start date (Monday) and end date (Sunday) of that week.
- 2. Date to ISO Parameters Conversion: Select any Gregorian date using the calendar picker to instantly find its corresponding ISO week number, ISO year, and numerical day of the week (1=Monday, 7=Sunday).
- 3. Validation & Leap Week Detection: The converter checks if the selected year contains 52 or 53 weeks, alerting you if week 53 is invalid for the chosen year and providing details about adjacent boundaries.
Key Rules of ISO 8601 Calendar Math
- The First-Thursday Rule: ISO Week 1 of any year is defined as the week containing the first Thursday of the Gregorian year. This is mathematically equivalent to the week containing January 4th.
- Cross-Year Overlaps: Because ISO weeks must start on Monday, the first few days of January can belong to Week 52 or 53 of the previous year, and the last few days of December can belong to Week 1 of the following year.
- Leap Week Years (53 Weeks): Most ISO years have 52 weeks (364 days). A year has 53 weeks (371 days) if it starts on a Thursday (common year) or if it starts on a Wednesday or Thursday (leap year).
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Frequently Asked Questions About ISO Weeks
What is an ISO week number?
An ISO week number is a standard index defined by the ISO 8601 standard for representing weeks. Weeks always start on Monday and are numbered from Week 01 to Week 52 or 53. It is widely used in global logistics, ERP setups, and financial operations.
When does ISO Week 01 start?
ISO Week 01 of any year is defined as the week that contains the first Thursday of January. This means January 4th is always in ISO Week 01. The starting Monday of W01 can fall between December 29th and January 4th.
How many weeks are in an ISO year?
An ISO year has either 52 or 53 weeks. Years starting on a Thursday (or a Wednesday in leap years) contain 53 weeks due to the accumulation of fractional weekdays over calendar years.
Why does the ISO week sometimes start in the previous Gregorian year?
Because ISO weeks must contain exactly 7 days starting on Monday, if January 1st falls on a Friday, Saturday, or Sunday, the days preceding it are part of the first week of the new year, pushing the start date back to late December.
How does this converter calculate date ranges?
The tool determines the starting Monday of the year using the "first Thursday" rule. It then multiplies the target week index minus one by 7 days to locate the start Monday and end Sunday of that week.
What is the format of an ISO week code?
The standard format is `YYYY-Www` (e.g. `2026-W23`). To specify a particular day of the week, a single digit is appended: `YYYY-Www-D` (e.g. `2026-W23-1` for Monday, `2026-W23-7` for Sunday).
Is this ISO week calculator free to use?
Yes! Our ISO Week to Date Converter is 100% free with no registration required. You can make unlimited conversions for any year from 1 to 9999.
Are these conversions processed online or offline?
All calculations are performed client-side in your web browser. No dates, years, or inputs are sent to remote servers, making the tool secure and accessible offline.