Week Number Calculator
Guide: Week Numbers and Countdowns: ISO Weeks, Business Days and Time Left
Nothing you enter or choose here leaves your device. This tool runs in your browser.
How to use
- 1Choose a date to see its week number, or switch to find the dates of a week.
- 2Read the ISO and US week numbers.
- 3See the day of the year, the quarter and the dates of that week.
Frequently asked questions
What is the ISO week number?
ISO weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. It is the standard used in Europe and in most business calendars.
How is the US week number different?
The US system starts weeks on Sunday and puts January 1 in week 1, so its numbers can differ from ISO by one.
Why can early January be in week 52 or 53?
Because ISO weeks follow the Thursday rule, the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next.
How many weeks does a year have?
52 or 53 ISO weeks. Years like 2020 and 2026 have 53.
Is my data sent anywhere?
No. Everything runs in your browser and nothing is sent to us.
Read the guide
Learn how this works and how to get the most from it.
Money, health & time
Week Numbers and Countdowns: ISO Weeks, Business Days and Time Left
How ISO and US week numbers work, why week 53 exists, how to count days and weekdays to a deadline, and where countdowns go wrong.
1 min read
Related tools
Date Calculator
Count the days between dates or add and subtract time from a date.
Countdown to a Date
Days, hours and minutes until any date. Live countdown.
Age Calculator
Find your exact age in years, months and days, and your next birthday.
Unix Timestamp Converter
Convert Unix epoch timestamps to dates and back, in any time zone.