Week Number Calculator
Week Number Calculator: free, accurate and easy to use. Works in your browser with no sign-up.
How to use
Find the ISO week number of any date, plus its day of the year, quarter, weekday and the number of days left in the year. ISO weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. Week numbers are widely used in business planning, schedules and logistics.
- Enter a date, time or value.
- Results update instantly.
- Copy the result you need.
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Frequently asked questions
How is the ISO week number defined?
Weeks run Monday to Sunday and week 1 is the week with the year’s first Thursday. Some days in early January or late December can belong to a neighboring year’s week.
Can a year have 53 weeks?
Yes. Some years have 53 ISO weeks, depending on which weekday January 1 falls on.
Is it free?
Yes. All tools are free, with no sign-up and no limits.
Is my data stored or sent?
No. Everything runs in your browser. A few tools remember your choices on this device only.
Are the results accurate?
Calculations use your browser’s built-in date and time zone data and standard formulas. For legal or religious dates, confirm with the official authority.