Week Number Calculator
Find a date’s ISO 8601 week number and its Sunday-start, US-style week number.
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Week Number Calculator
Find ISO 8601 and US-style week numbers.
Start Your Calculation
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Calculation Result
CalculatedHow to use the Week Number Calculator
- Select a calendar date.
- Click Calculate to see the ISO and US-style week numbers.
- Review the ISO week-year and Monday-to-Sunday week range.
How does this calculator work?
ISO 8601 weeks start Monday; week 1 is the week containing January 4 (equivalently, the first Thursday). ISO week-year can differ from the calendar year. The US-style week calculation in this tool starts weeks on Sunday and labels the week containing January 1 as week 1. Other US week numbering conventions exist, so confirm which convention your schedule uses.
Calculation example
January 1, 2021 is ISO week 53 of 2020 but week 1 under the Sunday-start, January-1-in-week-1 convention.
Frequently asked questions
What is ISO week 1?
It is the Monday–Sunday week containing January 4.
Why do ISO and US week numbers differ?
They differ in the first weekday and in how week 1 is assigned.
Can ISO week 1 start in December?
Yes. Some December dates are part of ISO week 1 of the following week-year.
Does this follow Excel WEEKNUM?
The Sunday-start convention is similar to Excel WEEKNUM with Sunday starts, while ISOWEEKNUM follows ISO rules.
Can a year have 53 ISO weeks?
Yes. Some ISO week-years have 53 weeks.