Week Number Calculator

Determine the official ISO-8601 week number, Monday start and Sunday end dates, 52 vs 53-week year totals, and ISO week-year codes for any calendar date.

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Calculations Guide

About the Week Number Calculator

The ISO Week Number Calculator delivers full ISO-8601 compliant calendar tracking for project managers, supply chain coordinators, and software developers. Select any calendar date to determine its official week number, ISO week-year, Monday starting date, and Sunday ending date. The tool also supports direct ISO week code lookups (such as '2026-W33'), identifies whether a given year contains 52 or 53 weeks, tracks remaining weeks in the year, offers step-by-step weekly navigation, and features an annual ISO progress bar.

Step-by-Step Guide

How to Use Week Number Calculator

1Pick any calendar date using the date selector, or click 'Set to Today' to check the current week.
2Alternatively, enter a designated ISO week code (e.g., '2026-W33') in the search bar and click 'Search Week'.
3View your primary ISO Week Result (e.g., 'Week 33 / 2026-W33').
4Inspect the designated Monday Start Date and Sunday End Date corresponding to that week.
5Navigate through adjacent weeks using the 'Previous Week' and 'Next Week' buttons.
6Copy the ISO week summary or share the date range report via device sharing.

Capabilities

Key Features & Highlights

Official ISO-8601 Compliance: follows international standards where Monday is the first day of the week and Week 1 contains the year's first Thursday.
ISO Week-Year Alignment: accurately resolves year-boundary dates where late December or early January days fall into adjacent ISO years.
52 vs 53 Weeks Calculator: determines the exact total number of ISO weeks (52 or 53) in any selected year.
Monday-to-Sunday Date Spans: clearly displays formatted start and end dates for the active week.
Direct ISO String Search: quickly look up date ranges by searching standardized codes like '2026-W33'.
ISO Year Progress Indicator: visual bar showing the percentage of the current ISO calendar year completed.
Private Client-Side Processing: all calendar calculations execute locally within your browser with zero data logging.
One-Click Summary Copy: copy the formatted ISO week output or share the result directly.

Practical Scenarios

Examples & Real-World Use Cases

ISO Week Calculation for August 15, 2026

Scenario: Identifying the official ISO week number for August 15, 2026.

Sample Input:

2026-08-15 (Saturday)

Expected Output:

ISO Week 33 (2026-W33) | Monday, Aug 10 to Sunday, Aug 16, 2026.

Outputs the exact ISO week code and full Monday–Sunday range.

Common Questions

Frequently Asked Questions

Under ISO-8601, weeks begin on Monday. Week 1 of any year is defined as the week containing the first Thursday of that calendar year (or the week containing January 4).

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