What is an ISO week?
An ISO week runs Monday to Sunday and is numbered from 1 to 52 or 53. The standard is ISO 8601, and it is the numbering most software and most of Europe uses.
Week Number Calculator
Find the ISO week number, day of the year and days left for today or any date you choose.
Runs in your browser — nothing is uploaded
About the Week Number Calculator
The ISO 8601 standard numbers weeks from Monday to Sunday. Week 1 of a year is the week containing the first Thursday, so the first days of January can belong to the last week of the previous year, and late December can belong to week 1 of the next.
Week numbers are used in planning, school timetables, payroll cycles and reporting. Leave the date empty to see today's values, or pick any date to check a deadline or event.
FAQ
Week Number Calculator — frequently asked
Why does the year number sometimes differ from the calendar year?
A date late in December can fall in week 1 of the next year, and a date in early January can fall in week 52 or 53 of the previous year. The calculator shows the ISO year next to the week for this reason.
How many weeks are in a year?
Most years have 52 ISO weeks, and some have 53. A year has 53 weeks when January 1 is a Thursday, or a leap year that starts on a Wednesday.
Is the US week numbering the same?
Not always. Many US systems start weeks on Sunday, and week 1 is the week containing January 1. Check which standard your organisation uses.
What is day of year used for?
Day of year, also called ordinal date, is common in scientific data, aviation and some programming systems. Day 1 is January 1, and day 365 or 366 is the last day.
Does the calculator use my local time?
It uses your local calendar date, so the week shown matches the day on your device. Time of day does not change the week.
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