Free Week Number Calculator
ISO 8601 week number for a given date.
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Inputs
Result
ISO week number
Week 1
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Short answer
ISO 8601 week number for a given date.
What is the Week Number Calculator?
ISO 8601 week number for a given date.
How does the Week Number Calculator work?
Enter the inputs above and read the result.
Formula
Variables
dtAdjusted date pointing to week's Thursday
Explanation
ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday.
Examples
Applications
Advantages
Limitations
Common mistakes
Tips
Related concepts
The Week Number Calculator sits inside the Date & Time Calculators hub, in the date arithmetic & calendars cluster. Adding to or classifying a date. Understanding the terms below makes the output easier to interpret and easier to compare against neighbouring measures.
Practical use cases and industry applications
Understanding Week Number
ISO 8601 week number for a given date. Within date & time calculators, week number belongs to the date arithmetic & calendars cluster, where it shares terminology and assumptions with closely related tools.
Learning how week number is calculated
ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday. Working through the variables one at a time — dt — makes the result reproducible by hand and easier to sanity-check.
Using week number to make a decision
Because outputs depend on the assumptions you enter, run more than one scenario before committing to a figure.
How week number compares with related measures
calendar rule, weekday, leap year, week number all describe adjacent aspects of date arithmetic & calendars. Comparing this calculator's output against those measures — using the related tools listed on this page — prevents a single metric from being read in isolation.
Frequently asked questions
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