Week Number Calculator
Find the ISO week number, day of year, day of week, and quarter for any date.
Overview
ISO 8601 defines week one of a year as the week containing its first Thursday, which is equivalent to the week containing 4 January. Weeks run Monday to Sunday, every week belongs entirely to one year, and a year therefore has either 52 or 53 ISO weeks. The definition is arithmetic rather than conventional, so any two systems that implement it agree exactly.
ISO Week Number
Day of Year
/
Day of Week
Quarter
Week Reference
How to Use
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1
Enter your values
Type your numbers into the Week Number Calculator input fields.
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Adjust the options
Choose the units, method, or region that fits your situation, or apply a quick preset.
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Read the result instantly
The result updates as you type — no submit button, and nothing is sent to a server.
About
The consequence is that early-January and late-December dates can belong to a neighbouring year's numbering. 1 January 2027 falls in week 53 of 2026, while 31 December 2024 falls in week 1 of 2025. Reporting systems that pair an ISO week number with the wrong calendar year misfile those boundary days, a defect that surfaces only once a year.
North American and Middle Eastern conventions differ: weeks there commonly start on Sunday or Saturday, and week one is the week containing 1 January regardless of weekday, so the same date can carry two different week numbers.