Week Number Calculator
Find ISO 8601 week numbers. Convert between dates and week numbers with both US and ISO week system support.
Week Number Calculator
ISO 8601 Week Number
Week 27
2026
ISO Week
Week 27
Year
2026
Day of Week
1 (Monday = 1)
Day of Year
180
Week Starts (Monday)
Monday, June 29, 2026
Week Ends (Sunday)
Sunday, July 5, 2026
What is Week Number Calculator?
Week Number Calculator handles the conversion between calendar dates and week numbers using both the international ISO 8601 standard and the US week numbering system two systems that disagree on roughly forty percent of weeks in any given year. Week numbering is surprisingly contentious and error-prone ISO 8601 standard defines week one as the week containing the first Thursday of the year or equivalently the week containing January fourth with weeks running Monday to Sunday the US system alternatively defines week one as the week containing January first with weeks running Sunday to Saturday and many businesses governments and institutions follow proprietary variants. This calculator clearly shows both systems side by side with the ability to toggle between ISO and US mode or display both simultaneously for comparison. Core functions include converting any date to its corresponding week number and year conversion in the opposite direction given a week number and year find both the Monday start and Sunday end date of that week display the current week number in real time when the page loads generate a full printable year calendar with all weeks numbered for any given year between 1900 and 2100 plus export calendar weeks for project planning. Automatically handles ISO week year discrepancies where late December early January dates belong to an ISO week year different from the Gregorian calendar year a major source of bugs in project management software.
When to Use Week Number Calculator
Use when referencing week numbers in project management tools Jira Asana Notion, scheduling meetings by week, converting week numbers from reports in different global systems, or verifying ISO week compliance in financial reporting.
How to Use Week Number Calculator
Select conversion direction date to week or week to date. Choose ISO 8601 standard or US convention system or compare both. Optionally generate full year view with numbered weeks for planning and export.
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